Effect of postgrowth annealing in an oxygen-containing atmosphere on the microhardness of single-crystal calcium molybdate CaMoO4

Abstract

Monocrystalline calcium molybdenum CaMoO4 is a well-known material. Recently, there has been a surge of interest in CaMoO4 due to a number of popular applications, such as working medium for a cryogenic scintillation bolometer. During growth CaMoO4 single crystals acquire a blue color due to the presence of defective centers, such as color centers, which is unacceptable for optical applications. To eliminate the coloration, annealing in an oxygen-containing atmosphere is used and then the necessary elements are prepared from the crystals by mechanical influences (cutting, polishing, etc.). In this regard, for the rational solution of issues arising in the manufacture of products from these crystals and their further practical use, the assessment of the mechanical properties of these crystalline materials is an urgent task. However, the results of studies of the mechanical properties of CaMoO4 are poorly presented, without taking into account anisotropy; there is a significant spread of data on the value of hardness by Mohs. For different authors, hardness varies from 3.5 to 6. In this paper, samples of single crystals of calcium molybdate in the initial state and after high-temperature annealing of different duration in an oxygen-containing atmosphere are studied. It is shown that prolonged annealing leads to discoloration of crystals. It has been established that calcium molybdate crystals are extremely brittle, the brittleness score of Zp crystals in the initial state is maximum and is 5, annealing leads to a decrease in the brittleness score to 4. The “viscosity” parameters are calculated by the Palmqvist S method. The nubs of complete destruction of Fpr prints were established, it was shown that annealing in an oxygen-containing atmosphere leads to an increase in Fpr by 2.5 times for the Z-cut, by 10 times for the X-cut. It is shown that the microhardness of crystals is characterized by anisotropy of the II kind: for all samples, the microhardness of the Z-cut is higher than the microhardness of the X-cut. The anisotropy coefficients of the microhardness of the KH samples are estimated. On the basis of the measured values of microhardness, the degrees of ionic bonds I are calculated.