Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics

Thermodynamic characteristics of the perovskite-like compound La₂/₃Ba₁/₃MnO₃ exhibiting structural phase transformation of the martensitic type with characteristic temperature Ts = 200 K have been studied in the temperature range 2–340 K. Step-like hysteretic temperature behavior of the effective...

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Datum:	2009
Hauptverfasser:	Beznosov, A.B., Fertman, E.L., Desnenko, V.A., Feher, A., Kajòaková, M., Ritter, C., Khalyavin, D.
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Veröffentlicht:	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2009
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Zitieren:	Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics / A.B. Beznosov, E.L. Fertman, V.A. Desnenko, A. Feher,M. Kajòaková, C. Ritter, D. Khalyavin // Физика низких температур. — 2009. — Т. 35, № 6. — С. 571–577. — Бібліогр.: 31 назв. — англ.

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spelling	nasplib_isofts_kiev_ua-123456789-1189702025-06-03T16:26:44Z Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics Beznosov, A.B. Fertman, E.L. Desnenko, V.A. Feher, A. Kajòaková, M. Ritter, C. Khalyavin, D. Низкотемпеpатуpный магнетизм Thermodynamic characteristics of the perovskite-like compound La₂/₃Ba₁/₃MnO₃ exhibiting structural phase transformation of the martensitic type with characteristic temperature Ts = 200 K have been studied in the temperature range 2–340 K. Step-like hysteretic temperature behavior of the effective heat capacity has been revealed at 150–250 K and attributed to the discrete kinetics and a latent heat of the martensitic transformation. Magnetic subsystem was found exhibiting magnetic glass state below 220 K and temperature hysteresis of the magnetic susceptibility brightly pronounced in the 40–100 K and 180–230 K regions. The Debye and Einstein temperatures, Ѳ D = 230 K and Ѳ E = 500 K, respectively, derived from the experimental Debye–Waller factors for La/Ba, Mn and O sublattices, have been used to refine contributions from the structural and magnetic transformations to the heat capacity and to reveal thermodynamically nonequilibrium states. The work was partly supported by grant from the National Academy of Sciences of Ukraine no. 3-026/2004 (Program «Nanosystems, nanomaterials, and nanotechnologies», contract no. 1/07-N) and by the Slovak Research and Development Agency, Grant no. 20-005204. 2009 Article Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics / A.B. Beznosov, E.L. Fertman, V.A. Desnenko, A. Feher,M. Kajòaková, C. Ritter, D. Khalyavin // Физика низких температур. — 2009. — Т. 35, № 6. — С. 571–577. — Бібліогр.: 31 назв. — англ. 0132-6414 PACS: 75.30.–m, 65.40.Ba, 81.30.Kf, 65.40.gd https://nasplib.isofts.kiev.ua/handle/123456789/118970 en Физика низких температур application/pdf Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
topic	Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм
spellingShingle	Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм Beznosov, A.B. Fertman, E.L. Desnenko, V.A. Feher, A. Kajòaková, M. Ritter, C. Khalyavin, D. Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics Физика низких температур
description	Thermodynamic characteristics of the perovskite-like compound La₂/₃Ba₁/₃MnO₃ exhibiting structural phase transformation of the martensitic type with characteristic temperature Ts = 200 K have been studied in the temperature range 2–340 K. Step-like hysteretic temperature behavior of the effective heat capacity has been revealed at 150–250 K and attributed to the discrete kinetics and a latent heat of the martensitic transformation. Magnetic subsystem was found exhibiting magnetic glass state below 220 K and temperature hysteresis of the magnetic susceptibility brightly pronounced in the 40–100 K and 180–230 K regions. The Debye and Einstein temperatures, Ѳ D = 230 K and Ѳ E = 500 K, respectively, derived from the experimental Debye–Waller factors for La/Ba, Mn and O sublattices, have been used to refine contributions from the structural and magnetic transformations to the heat capacity and to reveal thermodynamically nonequilibrium states.
format	Article
author	Beznosov, A.B. Fertman, E.L. Desnenko, V.A. Feher, A. Kajòaková, M. Ritter, C. Khalyavin, D.
author_facet	Beznosov, A.B. Fertman, E.L. Desnenko, V.A. Feher, A. Kajòaková, M. Ritter, C. Khalyavin, D.
author_sort	Beznosov, A.B.
title	Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics
title_short	Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics
title_full	Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics
title_fullStr	Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics
title_full_unstemmed	Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics
title_sort	low-temperature phase segregation in la₂/₃ba₁/₃mno₃: manifestation of nonequilibrium thermodynamics
publisher	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
publishDate	2009
topic_facet	Низкотемпеpатуpный магнетизм
url	https://nasplib.isofts.kiev.ua/handle/123456789/118970
citation_txt	Low-temperature phase segregation in La₂/₃Ba₁/₃MnO₃: Manifestation of nonequilibrium thermodynamics / A.B. Beznosov, E.L. Fertman, V.A. Desnenko, A. Feher,M. Kajòaková, C. Ritter, D. Khalyavin // Физика низких температур. — 2009. — Т. 35, № 6. — С. 571–577. — Бібліогр.: 31 назв. — англ.
series	Физика низких температур
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first_indexed	2025-11-26T08:17:10Z
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fulltext	Fizika Nizkikh Temperatur, 2009, v. 35, No. 6, p. 571–577 Low-temperature phase segregation in La2/3Ba1/3MnO3: Manifestation of nonequilibrium thermodynamics A.B. Beznosov, E.L. Fertman, and V.A. Desnenko B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine E-mail: beznosov@ilt.kharkov.ua A. Feher and M. Kajòaková Centre of Low Temperature Physics of the Faculty of Science of P.J. Šafárik University and IEP SAS, Park Angelinum 9, SK-04154 Košice, Slovakia C. Ritter Institut Laue-Langevin, Boite Postale 156, 38042 Grenoble Cedex 9, France D. Khalyavin Institute of Solid State and Semiconductor Physics NASB, 17 P. Brovka str., 220072 Minsk, Belarus Received December 5, 2008, revised February 17, 2009 Thermodynamic characteristics of the perovskite-like compound La2/3Ba1/3MnO3 exhibiting structural phase transformation of the martensitic type with characteristic temperature Ts � 200 K have been studied in the temperature range 2–340 K. Step-like hysteretic temperature behavior of the effective heat capacity has been re- vealed at 150–250 K and attributed to the discrete kinetics and a latent heat of the martensitic transformation. Magnetic subsystem was found exhibiting magnetic glass state below 220 K and temperature hysteresis of the magnetic susceptibility brightly pronounced in the 40–100 K and 180–230 K regions. The Debye and Einstein temperatures, �D = 230 K and �E = 500 K, respectively, derived from the experimental Debye–Waller factors for La/Ba, Mn and O sublattices, have been used to refine contributions from the structural and magnetic transforma- tions to the heat capacity and to reveal thermodynamically nonequilibrium states. PACS: 75.30.–m Intrinsic properties of magnetically ordered materials; 65.40.Ba Heat capacity; 81.30.Kf Martensitic transformations; 65.40.gd Entropy. Keywords: phase transitions, structural segregation, heat capacity, entropy, magnetic susceptibility, cluster glass. 1. Introduction Low-temperature phase segregation is characteristic feature of the perovskite-like complex manganese oxides [1]. As it was recently shown [2], structural segregation in manganites developes due to the martensitic transfor- mations (MT). The MT is a first-order diffusionless struc- tural transformation that generally occurs between a high-temperature parent phase (austenite) and a low-tem- perature phase with a lower symmetry (martensite). The transformation proceeds via an atomic rearrangement that involves a collective shear displacement. It is accompa- nied by the self-organized development of the phase segregated state in a wide temperature range, when the high temperature crystal phase and the low temperature one coexist. On cooling the growing low temperature phase applies stress to the other regions of the same crys- tallite (so-called accommodation strains), preventing them from transformation. Further cooling is needed for a further growing of the martensitic phase. The properties of such compounds are strongly dependent on the history of the material and experimental details [2,3]. The MT is © A. B. Beznosov, E. L. Fertman, V. A. Desnenko, A. Feher, M. Kajòaková, C. Ritter and D. Khalyavin, 2009 widely encountered in nature, with examples being ob- served in cuprate superconductors [4], transition metal al- loys [5], actinides [6], colossal magnetoresistance man- ganites [2,7–9]. They govern unique properties of the compounds, such as the shape-memory effect in intermetallic compounds [10], unusual stepwise magnetic relaxation [3,11] and the huge value of magnetoresistance [2] in manganites. The last effect stems from a combina- tion of the long-range deformations of the crystal lattice (accommodation strains) and the effects of strong elec- tron correlation [2]. MT in CMR perovskite manganites leads to the self-organized strain-induced nanome- ter-scale to sub-�m-scale phase coexistence [2,7,12]. Martensitic transformations revealed in the narrow-band CMR manganites are tightly connected with the charge ordering phenomena and lead to the hole undoped antiferromagnetic (AFM) and the hole rich ferromagnetic (FM) phase coexistence [2,13,14]. In wide band CMR manganites the structural transformations are often con- sidered a secondary effect and less studied. Meanwhile, in La2/3Ba1/3MnO3 the martensitic phase transformation in the vicinity of 200 K leads to a structural phase segre- gated state of the compound below room temperature, de- termining its characteristic properties. Among them there are the huge temperature hysteresis of magnetization and ultrasound properties, the giant anomaly of magnetic sus- ceptibility under low uniaxial pressure [8], stepwise tem- perature behavior of the magnetic susceptibility and the corresponding singular behavior of the internal friction [15]. The last phenomena reflect the specific relaxation phenomena caused by the discrete martensitic kinetics. It has to be mentioned, that the similar hysteretic behavior earlier reported for the low temperature structural phase transformations in the wide-band CMR manganites La0.8Âà0..2MnO3 [16,17], La0.825Sr0.175MnO3 [18,19] and La0.80Sr0.20MnO3 [18,20], permits to assume the martensitic nature of these transformations. Contrary to the martensitic transformations in the narrow band man- ganites reported, the MT in La2/3Ba1/3MnO3 occurs in the ferromagnetic state and leads to the coexistence of two ferromagnetic phases with equal electronic density: the rhombohedral R c3 (austenite) and the orthorhombic Imma one (martensite). 2. Experiment Here we use heat capacity and magnetic susceptibility measurements to gain insight into the nature of the martensitic phase transition in wide band manganites. A polycrystalline La2/3Ba1/3MnO3 was studied. The sam- ples were cut from ceramic pellets of the La2/3Ba1/3MnO3 compound which were prepared using standard so- lid-state reaction with stoichiometric amounts of powders of La2O3, BaCO3, and Mn2O3; the details are similar to those published in Ref. 8. The sample quality was con- firmed by x-ray diffraction study. Heat capacity measurements were made using Quan- tum Design Physical Properties Measurement System (PPMS) in cooling and heating mode in the 2–250 K tem- perature range. In order to ensure that the martensitic system had reached the quasi equilibrium state [21] it was allowed to stabilize for around one hour at each tempera- ture. Long stabilization times at each T point at a pinning of the structure domains boundaries to defects in the system [22,23]. The dc magnetization MN (and the ef- fective magnetic susceptibility �N = MN/H) of the La2/3Ba1/3MnO3 sample were measured in the temperature range 4–320 K in the external magnetic field H = 20 Oe by means of a SQUID magnetometer. The sample size was 3.13�2.39�3.14 mm, and the demagnetization factor, re- spectively, was N � 5 in the long side direction, along which the external magnetic field was applied. The effec- tive inner field in the sample was �0.4 Oe. Neutron dif- fraction data in the 5–370 K temperature range were ob- tained using the D2B diffractometer of the Institute Laue-Langevin at a wavelength of � = 1.594 � (the de- tails of the experiment are the same as those in Ref. 8). 3. Results and discussion The «effective heat capacity» value C(T) (Fig. 1,a) as measured by PPMS represents the sum of the heat capac- ity of the compound and the accompanying contribution caused by a latent heat of the first order (martensitic) phase transition (including shift energy of the structural domain boundaries). The C(T) data show an unusual non-regular step-like behavior in the region of martensitic phase transforma- tion around Ts � 200 K (Fig. 1,b). It coordinates with stepwise temperature behavior of the magnetic suscepti- bility and corresponding singular behavior of the internal friction in the R c3 � Imma transformation region which were found recently in the studied compound [15]. Such a kinetics is typical for reversible martensitic transforma- tions, when the macroscopic strain discontinuity splits into a set of bursts corresponding to transitions between neighboring metastable states [24]. It has to be noted, that martensitic structure develops as a result of a serial stepwise transformations of a microstructure, description of which require exploiting of methods of the none- quilibrium thermodynamics [25]. Elementary acts of the cooperative chain displacements of the coherent marten- sitic boundary, which proceed with a velocity of order of the sound one, represent characteristic feature of MT. Mi- croscopic theory of the phenomenon is based on the laser mechanism of MT, according to which diffusionless rear- rangement of a structure is provided by a coherent cou- pling of atoms due to the spontaneous emission of the phonons by the system, previously transferred into a 572 Fizika Nizkikh Temperatur, 2009, v. 35, No. 6 A. B. Beznosov, E. L. Fertman, V. A. Desnenko, A. Feher, M. Kajòaková, C. Ritter and D. Khalyavin nonequilibrium state. So, MT is not thermodynamic, but kinetic transition of the system «atoms+phonons» far from an equilibrium [21]. Thus, Fig. 1,b clearly reflects effects of such transitions on the cooling and heating pro- cesses in the system. Another characteristic feature of the phenomenon ob- served is an alternating hysteresis of the C(T) curves has been revealed above 50 K on cooling and heating (Fig. 2). The hysteretic behavior of the «effective heat capacity» found is consistent to that of the magnetic susceptibility �N above 40 K (Fig. 3). Below 40 K C(T) curves show no hysteresis within the experimental error. The hysteretic C(T) loop does not converge at 250 K. This can be con- sidered as an evidence of the incomplete reverse mar- tensitic transition. Significant uncertainty of the experimental data (char- acterizing nonequilibrium state of the system) is seen in the martensitic transformation region (Figs. 1,b, 2). This is in a good agreement with [23,26]: as passing through a first order transition, the sample should emit latent heat, producing a PPMS decay curve that cannot be well mod- eled by the analysis software. PPMS uses the thermal-re- laxation method based on the assumption that the sample specific heat does not vary over the temperature range en- compassed by a decay cycle. This will be true only when the relative change in the heat capacity over the tempera- ture span T is small. In the phase transition region the relative change in the sample heat capacity with tempera- ture can be large, which renders this assumption invalid. A decay cycle consists of the establishment of steady state at a temperature T0 + T (T0 is the thermostat tempera- ture, T ~ 0.02–0.2 K) followed by relaxation to T0. The time dependent temperature T(t) of the sample can be de- scribed as follows T t T T t ( ) exp � � � � � � �0 � , (1) where � is a thermal relaxation time. An extended series of decays cycles (10–100) has been averaged at each tem- perature to produce heat-capacity measurements that have a minimal point-to-point scatter depending on the temperature region. Temperature dependences of the decay time � were found to be anomalous (jump-like) above 50 K (Fig. 4). Such a peculiar character of the �(T) dependences in the martensitic transformation region reflects a latent-heat contribution associated with discontinuous burst-like martensit ic kinetics (the discrete process of the R c3 � Imma phases exchange exists in La2/3Ba1/3MnO3 in a wide temperature interval around Ts [8,15]). A similar non-regular behaviour of the temperature de- pendent heat capacity curves has been found in the shape-memory In-Tl alloy in the martensitic phase trans- Low-temperature phase segregation in La2/3Ba1/3MnO3 Fizika Nizkikh Temperatur, 2009, v. 35, No. 6 573 50 100 150 200 250 0 20 40 60 80 100 120 model cooling heating T, K 150 200 250 80 90 100 110 cooling T, K a b C , J/ g -m o l K� C * , J/ g -m o l K� Fig. 1. Temperature dependent «effective heat capacity» C(T) of La2/3Ba1/3MnO3 in cooling and heating mode (a). Stepwise behavior of C(T) in the structural transformation region (the dotted line is a guide for eyes). The solid lines represent the model Debye–Einstein heat capacity (b). 0 50 100 150 200 250 –4 –2 0 2 4 T, K C * – C * , J/ g -m o l K co o l h ea t � Fig. 2. Alternating temperature hysteresis of the «effective heat capacity» C(T) of La2/3Ba1/3MnO3 as difference of the data of cooling and following heating processes. formation region (Fig. 2 in [27 ]). It may be a common fea- ture of the temperature dependence of the effective heat ca- pacity of martensitic compounds in the vicinity of MT. The temperature dependent magnetic susceptibility �N(T) curves of La2/3Ba1/3MnO3 are strongly hysteretic below the room temperature (Fig. 3). As one can see in Fig. 3,b, there is an «additional» (to that between ~180 K and ~230 K [8,15]) pronounced temperature hysteresis of the magnetic susceptibility in the 40–100 K region, which implies a first order transition which was not re- ported earlier. Beside the difference between curves measured on heating and cooling there is a strong difference between the field cooled (FC) curves and zero field cooled ones (ZFC), which is typical for a cluster glass state. A similar behavior was previously reported for many phase segre- gated manganites close to a first-order electronic phase transitions [11,28]. The origin of the spin-glass-like char- acteristics is usually ascribed to the frustration introduced by the competition between ferromagnetic-double ex- change and antiferromagnetic-superexchange. In our case, below room temperature clusters of two ferromag- netic phases possessing different crystal structures are co- existing in different proportions due to the martensitic phase transition in the vicinity of 200 K [8]. The clusters are evidently interdependent by the martensitic accom- modation stresses. Characteristic glassy behavior and ac- companying relaxation phenomena (memory, aging, etc.) can be perfectly understood taking into account only the intercluster interactions [28]. One more peculiarity can be seen in the magnetic curves in the 100 K region (Fig. 3,a): a change of the slope of the ZFC �N(T) curve which seems to be connected with the above mentioned «additional» temperature hysteresis (Fig. 3,b) of the magnetic susceptibility in the low magnetic field 20 Oe. The change of the slope mentioned implies a decreasing of the number of the ferromagnetically ordered ions in the system. More complete description of the phe- nomenon requires a special research. In order to make this low temperature anomaly visible on the heat capacity curves as well the background was subtracted from the data for the last ones. The «pure lat- tice» heat capacity data (i.e. ones which do not include the phase transition and magnetic contributions, and so can be considered as certain background) were fitted using an equation of the form C T C T C TD Emod ( ) ( ) ( ) � , (2) where CD(T) and CE(T) are the Debye’s and Einstein’s contributions, respectively: 574 Fizika Nizkikh Temperatur, 2009, v. 35, No. 6 A. B. Beznosov, E. L. Fertman, V. A. Desnenko, A. Feher, M. Kajòaková, C. Ritter and D. Khalyavin 0 100 200 300 0,28 0,29 0,30 cooling heating ZFC H = 20 Oe T, K 0 50 100 150 0,297 0,298 0,299 0,300 cooling heating H = 20 Oe a b T, K � N � N Fig. 3. Temperature dependence of the effective magnetic sus- ceptibility �N of La2/3Ba1/3MnO3 in the external magnetic field 20 Oe at the field cooled and zero field cooled (ZFC) re- gimes (a); the 40–100 K hysteresis region (b). 0 50 100 150 200 250 20 40 60 80 100 120 cooling heating T, K �, s Fig. 4. Temperature dependence of the thermal relaxation time � (PPMS) of the La2/3Ba1/3MnO3 sample on cooling and heating. C T C T x dx D cl D x x T D ( ) . ( ) � �� 0 2 3 1 3 4 2 0 � � e e , (3) C T C T E cl E T T E E ( ) . � � � �� 0 8 1 2 2 2 � � � e e . (4) Both characteristic temperatures (the Debye temperature �D and the Einstein temperature �E) have been found from the experimental values of temperature dependent Debye–Waller factors B(T) (Fig. 5), obtained from the neutron diffraction data and fitted using the equations Eqs. (5)–(7) [29,30]: B T A T x xdxD D T D Mn ( ) coth� � � � � �� 2 3 0 2� � , (5) B T B A T x xdxs D D T D La/Ba ( ) coth� � � � � � �� 2 3 0 2� � , (6) B T A T E E E O( ) coth� � � 2 . (7) Here BMn(T), BLa/Ba(T) and BO(T) are experimental tem- perature dependent values of Debye–Waller factors for Mn, La(Ba) and O, correspondingly; AD = 29 � 2K, AE = = 445 � 2K and Bs = 0.237 � 2 are fitting parameters (Bs is a static contribution to the Debye–Waller factor caused by the chemical disorder in the La/Ba lattice sites), obtained by the least-squares method. We have found that in the studied temperature region 2–250 K a set with an acoustical mode of characteristic energy parameter �D = 230 K and an optical mode with �E = 500 K fit the «lattice background» (lattice heat ca- pacity without contributions from magnetic subsystem and structural phase transitions) fairly well (Fig. 1). The classical heat capacity is Ccl = 3R�, R is the universal gas constant, and � = 5 is the number of atoms in the formula unit of the compound. The coefficients 0.2 and 0.8 reflect weights of the three translation degrees of freedom of the formula unit and the twelve internal oscillatory ones, cor- respondingly. It has to be mentioned that the Debye temperature ob- tained �D = 230 K is much lower then ones being in use sometimes. For example the value �D � 400 K is obtained in the paper Ref. 31 using the low temperature data from 2–8 K region and Debye heat capacity C T C T x dx cl D x x T D ( ) ( ) � �� 3 1 3 4 2 0 � � e e in the low temperature limit for the lattice contribution. However value �D = 400 K being substituted into this equation permits neither to fit the temperature dependent heat capacity in the wide temperature range (and leads to the CD value substantially exceeding the experimental C(T) data between 50 K and 250 K), nor to fit the experi- mental temperature dependent Debye–Waller factors B(T) by Eqs. (5)–(7). Low-temperature phase segregation in La2/3Ba1/3MnO3 Fizika Nizkikh Temperatur, 2009, v. 35, No. 6 575 Oscillations of crystal lattice can be presented as oscillations of the unit cell as a whole (acoustical modes), and internal oscil- lations in the unit cell (optical modes). Oscillations of the formula units as a whole inside unit cells are determined by the same elastic modules, as the oscillations of the unit cells (chemical bonds between unit cells are the same as between formula units). So, it is reasonable to ascribe an acoustical spectrum to the formula units’ oscillations at a simplified approach, and to ascribe an optical spectrum to the internal oscillations of the formula units. Taking into account, that complete number of the degrees of freedom per formula unit is 15, three of which are translational ones (i.e. correspond to acoustical oscillations) and 12 de- grees of freedom correspond to optical oscillations, we get relation 0.2 to 0.8 for the corresponding contributions to the heat capacity per formula unit (or mole). 0 100 200 300 400 0,5 1,0 1,5 T, K O La/Ba Mn Fig. 5. Temperature dependent Debye–Waller factors B(T) of Mn, La/Ba and O in La2/3Ba1/3MnO3 compound obtained from neutron diffraction data. The dashed lines are approximations of the data by the Eqs. (5)–(7). The temperature dependence of the heat capacity data where the lattice contribution has been subtracted (Fig. 6) clearly shows two anomalous regions: the martensitic one around 200 K and the low temperature one below 100 K. The last one is evidently connected with the magnetic anomaly (see Fig. 3), nature of which requires an addi- tional study. It is clear, however, that the decreasing of the magnetization observed reflects an increasing of antiferromagnetic order parameter in certain regions inside of the structural domains or in the border layers between them (problem of determination of the character of the magnetic structure of La2/3Ba1/3MnO3 is discussed in [8]). Comparing the temperature dependence of the en- tropy, as calculated using the experimentally determined effective heat capacity S T C x x dx T ( ) ( ) � � 0 , with the model dependence S T C x x dx T mod mod( ) ( ) � 0 , its very unusual behavior was revealed (Fig. 7). The two anomalous regions found, A–B (75–118 K) and the cen- tral martensitic transformation region C–D (165–237 K), are characterized by having entropy values lower than the quasiequilibrium ones. This indicates that in the phase transformation regions the steady state has not been achieved during the thermal relaxation time � (~100 s) used in the present heat capacity experiment (Fig. 4). 4. Conclusion In conclusion, we have studied the heat capacity and magnetic behavior of the La2/3Ba1/3MnO3 perovskite in the phase segregated state (when two different crystal 576 Fizika Nizkikh Temperatur, 2009, v. 35, No. 6 A. B. Beznosov, E. L. Fertman, V. A. Desnenko, A. Feher, M. Kajòaková, C. Ritter and D. Khalyavin 0 100 200 300 0 5 10 cooling 0 100 200 300 0 5 10 heating T, K T, K a b C * – C , J/ g -m o l K co o l m o d � C * – C , J/ g -m o l K h ea t m o d � Fig. 6. Temperature dependent «effective heat capacity» data with model lattice contribution subtracted on cooling (a) and on heating (b). 100 200 300 0 50 100 cooling heating model 100 200 300 0 5 10 15 20 D C B A cooling heating T, K T, K a b S , J/ g -m o l K� S – S , J/ g -m o l K m o d � Fig. 7. Entropy of La2/3Ba1/3MnO3 when cooling (�) and heat- ing (�); solid line stands for the model dependence Smod(T) (a). Difference between the experimental and model dependences. The A–B and C–D intervals correspond to the nonequilibrium states in the experiment. Dotted line is guide for eyes (b). structures are coexistent, both being ferromagnetic), which develops due to the martensitic phase transforma- tion below the room temperature. The temperature de- pendent «effective heat capacity» C*(T) was found to be hysteretic and step-like in the martensitic transformation region 150–250 K. The temperature dependent decay time � is found to be non-regular jump-like as well. The behavior revealed reflects a latent-heat contribution asso- ciated with the discrete burst-like martensitic kinetics of the transformation. The temperature behavior of the en- tropy permits to conclude that the steady (i. e. thermody- namically quasiequilibrium) state is not achieved during the thermal relaxation time � (~100 s). The lattice part of the heat capacity was modeled by the sum of the Debye and the Einstein contributions in the whole studied tem- perature interval 2–250 K. The Debye temperature �D = = 230 K and the Einstein temperature �E = 500 K used have been obtained from the experimental temperature dependent Debye–Waller factors. Magnetic subsystem of the compound exhibits magnetic glass state below 220 K and «additional» temperature hysteresis of the magnetic susceptibility in the 40–100 K region. 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Low-temperature phase segregation in La2/3Ba1/3MnO3 Fizika Nizkikh Temperatur, 2009, v. 35, No. 6 577

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