The characteristics of the electronic and phonon subsystems of lanthanum tetraboride (LaB4) studied using first-principles calculations. The calculated lattice parameters, as well as the positions of ...atomic, are satisfactorily consistent with the experimental data. The partial densities of states, band structure, Fermi surface, phonon dispersion curve of LaB4 are calculated and analysed. The reliability of the calculation results is confirmed by a satisfactory agreement between the calculated thermodynamic parameters of LaB4 (temperature changes in heat capacity, entropy, Grüneisen parameter and volume modulus) with experimental data.
•The electronic and phonon properties of LaB4 studied by first-principles band theory.•The experimental data of specific heat and lattice parameters at 2–300 K are analysed.•The calculated thermodynamic parameters of LaB4 correspond to experimental data.
•Temperature changes of cell parameters Eu₂Cu₆P₅ revealed anisotropy at 5–300 K.•Kinks on a(T) and c(T) curves at TC ≈ 36 K are due to magnetic phase transition.•The parameters of the Debye-Einstein ...model for cell volume Vu(T) were determined.•Grüneisen parameter growth with temperature is due to impact of anharmonicity.
Ternary europium copper phosphide Eu₂Cu₆P₅ was synthesized from the elements by the standard ampule method as a phase-pure product and studied by means of X-ray powder diffraction. Temperature dependencies of the tetragonal unit cell parameters and volume revealed pronounced anisotropy. The observed anomalies on the a(T) and c(T) curves at about TC ≈ 36 K were related to magnetic phase transition in the phosphide under study. The linear spontaneous magnetostrictions in the basal plane and in the direction of the c axis have different signs that indicates a pronounced anisotropy of the thermal expansion at the temperatures of magnetic ordering. Temperature changes of the unit cell volume Vu(T) were analyzed within the Debye-Einstein approximation. The parameters of the model applied (Debye and Einstein characteristic temperatures, shares of the correspondence contributions) were determined. It was shown that the ratio of the shares of the Debye and Einstein contributions to the thermal characteristics are 0.85/0.15; consequently, in the region of moderately low temperatures, the thermal properties of Eu2Cu6P5 are determined mainly by the Debye vibrations of the Cu/P framework. Experimental data on thermal expansion and calculated heat capacity were used to calculate the temperature-dependent Grüneisen parameter, γ(T), for the further analysis of the lattice dynamics of Eu2Cu6P5. The revealed monotonic growth of γ(T), which is a measure of the anharmonicity of lattice vibrations, indicates an increasing influence of anharmonicity on the thermal properties of Eu₂Cu₆P₅. The anisotropy of the vibrations of europium atoms, as well as local regions of structural heterogeneity of the sample that leads to the appearance of local stresses in the crystal structure can be the reasons for the increasing influence of anharmonicity.
•The experimental data of LaF3 and (SrF2)0.5(LaF3)0.5 heat capacity at 2–300 K are analysed.•The experimental lattice parameters of LaF3 and (SrF2)0.5(LaF3)0.5 at 5–300 K are analysed.•The anomalies ...of studied fluorides properties are due to their lattice disorder.
Temperature dependences of the heat capacity Cp(T) (2–300 K) and lattice parameter a(T) (5–300 K) of LaF3 and (SrF2)0.5(LaF3)0.5, in addition to the earlier studied SrF2, are experimentally investigated for the first time. The experimental dependences of Cp(T) and the unit cell volume V(T) of the studied fluorides are analysed using the Debye–Einstein model considering the influence of the disorder of their crystal structures. The parameters of the model are determined. An anomalous arrangement of the Cp(T) and V(T) curves of the solution with respect to the corresponding curves of the components is revealed. At low temperatures (T < 150 K), the values of Cp(T) and V(T) of the solution exceed the values calculated by the additivity rule. At higher temperatures (T > 150 K), deviations towards smaller values are observed. We have established that the disorder in solid-solution crystal structure is the common reason of anomalies revealed. It is found that at low temperatures, the cause of these anomalies is a decrease in the vibrational frequencies in the solid-solution metal sublattice upon the replacement of Sr with La, which has a large size and mass. Such a replacement leads to distortions of the original structure and an increase in its disorder. At elevated temperatures, the deviations are caused by an increase in the vibrational frequencies in the F- sublattice due to distortion of the crystal structure, as well as the increased influence of the anharmonicity of the disordered crystal lattice vibration.
The temperature dependence of the heat capacity Cp(T), (2–300 K), and the lattice parameters a(T), b(T), and c(T) (5–300 K) TmF3 were experimentally studied. Anomalies in the studied characteristics ...were identified due to the influence on the thermal properties of trifluoride of the crystal field, and at the lowest temperatures (2–10 K), of a possible glass-like contribution due to disorder in the crystal lattice. As a result of our analysis of the excess contribution to the heat capacity (Schottky contribution, CSch(T)), we proposed a simplified scheme based on 1-1-7-5 splitting of the ground state of the Tm3+ ion, and determined the splitting parameters. The influence of the crystal field is also manifested through anomalies in the thermal expansion of TmF3. The temperature dependences of the unit cell volume of thulium trifluoride, and its heat capacity, were analysed using the Debye-Einstein model, and the model parameters are determined.
•The specific heat of TmF3 at 2–300 K has been analysed.•The lattice parameters of TmF3 and at 5–300 K are analysed.•The anomalies of studied fluoride properties are due to lattice disorder.
A homogeneous powder sample of EuCu2P2 was synthesized from the elements by a two-step high-temperature ampule technique. X-ray powder diffraction analysis have shown that the compound belongs to the ...ThCr2Si2 structure type and features copper deficiency. The study of temperature changes of heat capacity, Cp(T) (2–300 K), and unit cell parameters, a(T) and c(T) and volume Vu(T) (5–300 K), revealed a pronounced anomaly at about TСmах = 44 K evidencing the phase transition to the ferromagnetic state. Detailed analyses of the extracted excess (magnetic) components of EuCu2P2 thermodynamic characteristics, Cm(T) and ΔVm(T), have been performed using the Debye-Einstein and mean field theory approaches. We have found that the anomalous nature of Cm(T) and ΔVm(T) at T < TСmах is caused by the effect of the internal magnetic field on the magnetic moments of Eu2+ ions, and, to a lesser extent, by the splitting of the ground state of Eu2+ ions. Noticeable anisotropy of the EuCu2P2 thermal expansion at temperatures below TCmax as well as a significant effect of the anharmonicity of lattice vibrations on the thermal properties of EuCu2P2 at elevated temperatures have been documented.
•Heat capacity and thermal expansion of the EuCu2P2 were studied at 2–300 K.•Anomalies of thermal characteristics due to magnetic phase transitions were analyzed.•Anisotropy and anharmonicity of lattice vibrations have been documented.
The heat capacity and lattice parameters of europium fluoride EuF2.16 and (SrF2)0.5(EuF2.16)0.5 solid solution are experimentally investigated at temperatures 2–300 K. The revealed anomalies in the ...heat capacity are due to magnetic phase transition and disorder in fluorides crystal structures. The dependencies studied were analyzed using the Debye–Einstein approximation, taking into account the influence of the magnetic phase transition, two-level systems, and anharmonicity of lattice vibrations on their thermodynamic properties. We determined the parameters of the Debye and Einstein models – the characteristic temperatures as well as the magnitudes their respective contributions to the overall thermodynamic characteristics, the parameters characterizing crystal structure disorder, and the exchange parameters characterizing interaction in magnetic subsystems of fluorides. Dependences studied for solid solution deviate from the additive values. We further established that the mechanisms causing these are based on the disorder in the crystal structures of europium fluoride and its solid solution.
•The specific heat of EuF2.16 and (SrF2)0.5(EuF2.16)0.5 at 2–300K are analyzed.•The lattice parameters of EuF2.16 and (SrF2)0.5(EuF2.16)0.5 at 5–300K are analyzed.•The anomalies of studied fluorides properties are due to their lattice disorder.
The temperature changes in the heat capacity Cp(T) and lattice parameters a(T) of solid solutions of CaxSr1-x F2 are experimentally studied at temperatures of 2–300 K for x = 0, 0.25, 0.5, 0.75 and ...1. Deviations from the Kopp-Neumann rule (Vegard's rule) in the values of Cp(T) and a(T) for these solutions are identified.
The experimental dependencies of Cp(T) and a(T) for end-member crystals and their solid solutions are analysed using the Debye-Einstein model, taking into account the effects of anharmonicity of lattice vibrations at elevated temperatures, and the parameters of this approximation are determined. An anomalous arrangement of the curves for Cp(T) and a(T) is revealed for these solutions, relative to the corresponding dependence for end-member crystals. Despite cationic substitution taking place during the formation of solid solutions of the system under study, restructuring of the crystal structure mainly affects the nature of the oscillations of the anion sublattice. Most affected by the anharmonicity are the Einstein modes, corresponding to the vibrations of the F− ions, and the coupled vibrations of two different fluorine sublattices.
•The heat capacity and lattice parameters of CaxSr1-x F2 are studied at 2–300 K.•Deviations from the Kopp-Neumann rule for these solutions are identified.•The dependencies of Cp(T) and a(T) are analysed using the Debye-Einstein model.•The effects of anharmonicity of lattice vibrations are determined.
A holmium boride HoB50 polycrystalline sample was synthesised by boron-thermal reduction from the elements at high temperature. The temperature dependencies of the heat capacity Ср(Т) and the unit ...cell parameters a(T), b(T), c(T) of holmium boride were determined experimentally in the temperature range 2–300 K. In terms of Cp(T) dependence, smooth anomalies were revealed at TС1 max ≈ 6.5 K and TС2 max ≈ 30 K; the first of these is caused by an antiferromagnetic transition and the second by the influence of the crystalline electric field (CEF). The high-temperature anomaly in the heat capacity of the boride is satisfactorily reproduced due to a simplified scheme for CEF-splitting of the ground state of the Ho3+ ions.
The dependencies a(T), b(T), c(T) of holmium boride clearly show a pronounced anisotropy of thermal expansion. The anomalies in the temperature changes of the HoB50 unit cell volume at elevated temperatures are analysed within the CEF model described above.
•Heat capacity Ср(Т) and thermal expansion V(T) of the HoB50 boride were studied in the temperature range of 2–300 K.•A large range of low temperatures negative thermal expansion caused by defects in the crystal lattice was revealed.•The phase transition of HoB50 to the antiferromagnetic state appears as broad anomaly of the heat capacity at about 5.8 K.•The effect of the crystal electric field on the thermal characteristics of HoB50 was observed.
•Thermal and magnetic properties of DyB50 were studied at 2–300K.•A heat capacity and magnetic susceptibility show the anomalies at about TN≈6.3K.•The anomalies of DyB50 properties at higher ...temperatures are due to the CEF.•The scheme of CEF-splitting of Dy3+ ion ground level in boride has been proposed.
The specific heat, thermal expansion and magnetic properties of polycrystalline sample of nonstoichiometric dysprosium boride DyB50, that was synthesized by boron-thermal reduction of the metal from its oxide in vacuum, were studied at 2–300 K. The measurements revealed a heat capacity and magnetic susceptibility anomalies at about TN ≈ 6.3 K, caused by the transition to an antiferromagnetic state. The anomalies of studied properties of DyB50 at higher temperatures have been ascribed to the influence of crystal electric field (CEF). The scheme of CEF-splitting of Dy3+ ion ground level in boride has been proposed.
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