Piezoelastic properties of retgersite determined by ultrasonic measurements

The elastic tensor of the tetragonal NiSO4 . 6H2O (retgersite) was measured with a resonant ultrasonic plane parallel plate technique at room temperature as a function of hydrostatic pressure in the range of 0.1-50 MPa and the piezoelastic coefficients have been derived. The monotonic increase in the cij shows that retgersite does not undergo a phase transition in this pressure range. Density functional theory (DFT) based calculations within the linear response method were used to predict the variation of the cij in the range of 1-1000 MPa in the static limit. A comparison of the two data sets shows that while structural parameters and elastic stiffness coefficients are well reproduced by the DFT model, theoretical piezoelastic coefficients of compressible compounds are only in moderate agreement with the corresponding experimental values. The limitations of DFT-based calculations for the calculation of piezoelastic coefficients are discussed

Physical Properties of La 1−x_{1− x} Eu x_{x} PO 4_{4} ,0 ≤ x ≤ 1, Monazite-Type Ceramics

Synthetic La1−xEuxPO4 monazite-type ceramics with 0 ≤ x ≤ 1 have been characterized by ultrasound techniques, dilatometry, and micro-calorimetry. The coefficients of thermal expansion and the elastic properties are, to a good approximation, linearly dependent on the europium concentration. Elastic stiffness coefficients range from 182(1) to 202(1) GPa for c11 and from 53.8(7) to 61.1(4) GPa for c44. They are strongly dependent on the density of the sample. The coefficient of thermal expansion at 673 K is 8.4(3) × 10−6 K−1 for LaPO4 and 9.9(3) × 10−6 K−1 for EuPO4, respectively. The heat capacities at ambient temperature are between 101.6(8) J·(mol·K)−1 for LaPO4 and 110.1(8) J·(mol·K)−1 for EuPO4. The difference between the heat capacity of LaPO4 and the Eu-containing solid solutions is dominated by electronic transitions of the 4f-electrons at temperatures above 75 K