THE LOW-TEMPERATURE AND HIGH-PRESSURE THERMOELASTIC AND STRUCTURAL PROPERTIES OF CHALCOPYRITE, CuFeS2

The thermoelastic properties of a sample of chalcopyrite from the Palabora mine, South Africa, have been investigated in the temperature range 4.2 – 330 K at ambient pressure, and between 0.22 and 6.81 GPa at ambient temperature. Magnetization measurements indicated a transition to a second antiferromagnetically ordered phase in the region of 53 K; however, all attempts to characterize this magnetic phase by introducing an ordered moment onto the copper site were unsuccessful owing to the small magnitude of the refined magnetic moment. In agreement with other low-temperature crystallographic measurements made on non-antiferromagnetically ordered adamantine-structured semiconducting materials (group IV; I–VII, II–VI, III–V, I–III–VI2, II–IV–V2 compounds), chalcopyrite exhibits negative linear and volumetric thermal expansion over a significant temperature interval. Calculation of the speeds of sound for a number of high-symmetry wave vectors is consistent with Blackman’s model for negative thermal expansion. The unit-cell volume and isochoric heat-capacity have been fitted assuming a two-term Debye internal energy function, with consistent values being found for the two characteristic temperatures. The temperature dependence of the thermodynamic Gruneisen parameter shows a deep minimum of ~−3 at T/θ ~0.55 (θ is the Debye temperature at 0 K) and a high-temperature limit of ~0.7; these results are the first demonstration that a chalcopyrite-structured phase behaves in the characteristic manner of the simpler adamantine-structured semiconducting materials. No systematic variation in either the nuclear nor the magnetic structure was found between 4.2 and 330 K, and the vibrational Debye temperatures derived by fitting the temperature dependence of the isotropic atomic displacement parameters show no relationship to features in the phonon density of states function. The bulk modulus of chalcopyrite is 77(2) GPa, in good agreement with that determined by ab initio calculations and a recent X-ray-diffraction study, and its pressure derivative is 2.0(6). At high pressure, chalcopyrite remains antiferromagnetically ordered until 6.7(2) GPa, at which point a transition to an amorphous phase is observed. Slow decompression of this phase leads to only a limited recovery of the crystalline phase.