X-ray diffraction study of phonon and magnon properties of Eu2Cu6P5 ferromagnet

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

Electronic and thermodynamic properties of lanthanum tetraboride on low-temperature experimental and ab-initio calculation data

*Özışık, Hacı ( Aksaray, Yazar ) *Deligöz, Engin ( Aksaray, Yazar )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, Gruneisen parameter and volume modulus) with experimental data

The specific features of phononic and magnetic subsystems of type-VII clathrate EuNi2P4

A type-VII clathrate with a Eu(2+)guest embedded into a Ni-P covalent framework, EuNi2P4, was synthesized by a standard two-stage ampoule synthesis and confirmed to crystallize in the orthorhombic space groupFdddwith unit cell parametersa= 5.1829(1) angstrom,b= 9.4765(1) angstrom, andc= 18.9900(1) angstrom. A general technique for studying the lattice and magnetic properties of REE containing compounds is proposed. The temperature and field dependences of electrical resistivity rho(T,H), magnetizationM(T,H), magnetic susceptibility chi(T,H), heat capacityC(p)(T), and unit cell parametersa(T),b(T),c(T), and volumeV(T) were experimentally studied and analyzed at different pressures in the temperature range of 2-300 K. A cascade of anomalies in the studied dependences was identified and attributed to the magnetic phase transformation and peculiar lattice contributions at temperatures below 20 K. As a result of comparison with an isostructural clathrate SrNi2P4, the parameters of the magnetic and lattice contributions were determined. It is characteristic that the phase transition from the paramagnetic to the magnetically ordered state is not reflected in the temperature changes of the lattice parameters due to weak bonds between guest europium atoms and the Ni-P host matrix. We have constructed a tentativeH-Tphase diagram based on theM(T) andM(H) data, which includes 6 different phases. It is established that the anomalous lattice contribution to the clathrate heat capacityC(TLS)(T) appears due to the effect of two-level systems (TLS) in the Eu(2+)subsystem on the thermodynamic properties of EuNi2P4. The values of TLS parameters as well as the parameters of the magnetic subsystem of the clathrate were determined

Ferromagnetic phase transition and anomalies of thermodynamic characteristics of copper-deficient EuCu2P2 at low temperatures

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 V-u(T) (5-300 K), revealed a pronounced anomaly at about T-Cmax = 44 K evidencing the phase transition to the ferromagnetic state. Detailed analyses of the extracted excess (magnetic) components of EuCu2P2 thermodynamic characteristics, C-m(T) and Delta(Vm)(T), have been performed using the Debye-Einstein and mean field theory approaches. We have found that the anomalous nature of C-m(T) and Delta(Vm)(T) at T < T-Cmax. 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. (c) 2020 Elsevier B.V. All rights reserved