Heat capacity, thermal expansion and sensitivity to hydrostatic pressure of (NH4)3SiF7 at successive structural phase transitions

Heat capacity, thermal dilatation, permittivity and sensitivity to external pressure of (NH4)3SiF7 were studied. Due to the absence of cubic phases Pm3m and Pa3, a strong decrease in the total entropy change dSi = 19 J/molK associated with four successive transformations P4=mbm - Pbam - P21=c11 - P1 - P121/c1 was found in silicate in comparison with other double uoride salts (NH4)3MeF7 (Me: Ge, Ti, Sn). Using analysis of the excess heat capacity in the framework of the thermodynamic theory, the entropies associated with each individual phase transition were determined. In accordance with the entropic parameters, the complete ordering of the structural elements occurs in the monoclinic phase P21=c11. Further change in symmetry is associated with small entropy changes which prove insignicant displacement of structural units. A T-p phase diagram was constructed and good agreement was found between measured and calculated baric coefficients

Conventional and inverse barocaloric effects in ferroelectric NH4HSO4

In this study, the conventional and inverse barocaloric effects (BCE) in ferroelectric NH4HSO4 are reported. Maximum extensive and intensive BCE near orderedisorder phase transition can be achieved at low pressure p < 0.1 GPa. Large thermal expansion of the crystal lattice plays a very important role in the developing conventional BCE and conversation between BCE of different sign in the narrow temperature range

T − p phase diagrams and barocaloric effect in materials with successive phase transitions

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Abstract An analysis of the extensive and intensive barocaloric effect (BCE) at the successive structural phase transitions in some complex fluorides and oxyfluorides was performed. A high sensitivity of these compounds to a change of the chemical pressure allows one to vary successions and parameters of the transformations (temperature, entropy, baric coefficient) in wide ranges and obtain the optimal values of BCE. A comparison of different types of the schematic T − p phase diagrams with the complicated T(p) dependences observed experimentally shows that in some ranges of temperature and pressure BCE in compounds undergoing successive transformations can be increased due to summation of caloric effects associated with the distinct phase transitions. The maximum values of the extensive and intensive BCE in complex fluorides and oxyfluorides can be realized at rather low pressure, 0.1 – 0.3 GPa. In narrow temperature range around triple points the conversion from the conventional BCE to the inverse one is observed which is followed by gigantic change of both |∆SBCE| and |∆TAD|

Optical and calorimetric studies of K2TaF7

Optical and calorimetric experiments on K2TaF7 are performed in a wide temperature range. No features were found in the behavior of the birefringence Δnb(T), the angle of rotation of the indicatrix ϕ(T) and the heat capacity ΔCp(T) except for those associated with the Pnma↔P21/c phase transition. Structural transformation was characterized as strong first order “proper” ferroelastic accompanied by a huge angle ϕ ≈40° and strong pretransition phenomena in Δnb(T). Two contributions to the anomalies of the optical properties were found associated with the photoelastic effect and the transition parameter related linearly to the spontaneous deformation. Thermal treatments cause correlated changes in temperature and enthalpy of the phase transition, which leads to the invariance of the large magnitude of the corresponding entropy ΔS=22 J/mol·K which does not match the model with the absence of structural disorder in the Pnma phase

Anomalous behaviour of thermodynamic properties at successive phase transitions in (NH4)3GeF7

Heat capacity, thermal dilatation, susceptibility to hydrostatic pressure and dielectric properties associated with succession of three phase transitions below room temperature in double fluoride salt (NH4)3GeF7 were studied. A possible transformation into the parent Pm-3m cubic phase was not observed up to the decomposition of compound. Nonferroelectric nature of structural distortions was confirmed. The DTA under pressure studies revealed a high temperature stability of two phases: P4/mbm and Pbam. The entropies of the phase transitions agree well with the model of structural distortions. Analysis of the thermal properties associated with the individual phase transitions in the framework of thermodynamic equations has shown a high reliability of the data obtained

Synthesis of NdSc3(BO3)4 single crystals and study of its structure properties

Using the group method, single crystals of NdSc3(BO3)4 are grown from a melt solution and X-ray structural studies are performed. It is shown that at room temperature the NdSc3(BO3)4 crystal has a huntite-type structure with space group P3121. The temperature dependence of the heat capacity shows anomalous behaviour at Т ¼ 504 ± 1 K. This anomaly corresponds to a structural phase transition from R32 to P3121. It is known that a similar transition occurs in RFe3(BO3)4 crystals; an anomaly in the specific heat is also observed. Theoretical calculations are carried out from the first principles of the lattice dynamics of the crystal under study in a high-symmetry phase with the R32 space group. An unstable (soft) mode in the boundary point of Brillouin zone was found. It was determined that this structural instability is responsible for the structural displacement-type phase transition R32 / P3121

Современная практика искусственного лесовосстановления в таежной зоне европейской части России

Приводится анализ технологий и приемов создания лесных культур, применяемых в Архангельской, Кировской областях и Республике Коми, по материалам проектов лесовосстановления за 2020 г. Рассмотренные территории относятся к 6 лесным районам: Северо-таежному, Двинско-Вычегодскому таежному, Южно-таежному, хвойно-широколиственных (смешанных) лесов, Западно-Уральскому таежному и району притундровых лесов и редкостойной тайги. Созданная на основе данных проектов лесовосстановления база данных включает в себя 13 показателей: категории площадей лесовосстановления, лесорастительные условия (рельеф, тип почв, влажность почвы, группа типов лесов, степень задернения почвы), а также способы обработки почв, механизмы и агрегаты, применяемые для обработки почвы и посадки сеянцев и саженцев, вид посадочного материала. Установлено, что в изученных регионах лесовосстановление чаще проводится на свежих (1–2-летних) вырубках из-под еловых насаждений черничного типа леса, с подзолистыми дренированными и слабодренированными почвами. При обработке почвы под лесные культуры широко используются экскаваторы (44 %), заменяющие традиционные трактора с плугами (чаще ПЛ-1). На большинстве площадей обработку почвы выполняют бороздами (52 %), но распространены также полосная обработка, микроповышениями и площадками (40 %). Больше стали использовать посадочный материал ели и сосны с закрытой корневой системой (> 50 % площадей), при этом саженцы встречаются крайне редко (7 % в Кировской области). Полученные данные позволяют оценить реальную картину применения технологий для лесовосстановления в таежной зоне на примере 3 субъектов Российской Федерации, что поможет при принятии управленческих решений для осуществления субъектами РФ своих полномочий в сфере лесовосстановления

Thermal, structural, optical, dielectric and barocaloric properties at ferroelastic phase transition in trigonal (NH4)2SnF6: A new look at the old compound

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Structural phase transition P-3m1 ↔ P-1 of the first order and nonferroelectric nature was found in (NH4)2SnF6 at about 110 K in the process of thermal, optical, dielectric and X-ray studies. Order-disorder transformation is accompanied by large changes of entropy (ΔS0 = 16.4 J/mol•K) and volume (ΔV0/V ≈ 1 %). The structural model associated with three orientations of NH4 tetrahedra in P-3m1 phase and their complete ordering in P-1 phase was suggested with the entropy change (ΔS0 = 18.3 J/mol•K) close to the experimental value. A good agreement between baric coefficients measured in a direct way (dT0/dp = -157 K/GPa) and evaluated using entropy and volume changes at phase transition (- 170 K/GPa) was found. Barocaloric effects in (NH4)2SnF6 are comparable with those of known solid state refrigerants and are characterised by a low-pressure need for realising their maximum values

Thermal, structural, optical, dielectric and barocaloric properties at ferroelastic phase transition in trigonal (NH4)2SnF6: A new look at the old compound

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Structural phase transition P-3m1 ↔ P-1 of the first order and nonferroelectric nature was found in (NH4)2SnF6 at about 110 K in the process of thermal, optical, dielectric and X-ray studies. Order-disorder transformation is accompanied by large changes of entropy (ΔS0 = 16.4 J/mol•K) and volume (ΔV0/V ≈ 1 %). The structural model associated with three orientations of NH4 tetrahedra in P-3m1 phase and their complete ordering in P-1 phase was suggested with the entropy change (ΔS0 = 18.3 J/mol•K) close to the experimental value. A good agreement between baric coefficients measured in a direct way (dT0/dp = -157 K/GPa) and evaluated using entropy and volume changes at phase transition (- 170 K/GPa) was found. Barocaloric effects in (NH4)2SnF6 are comparable with those of known solid state refrigerants and are characterised by a low-pressure need for realising their maximum values