Top seeded solution growth of KHo(WO4)2 single crystals

Monoclinic KHo(WO4)(2) single crystals have been grown by means of the Top Seeded Solution Growth method from K2W2O7 self-flux containing up to 30mol% of the crystallized compound. KHo(WO4)(2) seed was selected from small crystals obtained by spontaneous crystallization and used to grow the first bulky KHo(WO4)(2) crystal, from which seeds perpendicular to {111} and {110} planes, used in subsequent experiments, were cut out. Crystals grown on these seeds are designated in this paper as 111 and 110 crystals, respectively. Properly chosen thermal conditions allowed growth of crystals with one dominating crystallographic plane forming the bottom of the 111 crystals. Under conditions of very low temperature gradients, growing crystals developed bottoms consisting of several crystallographic planes, while a slightly steeper temperature gradient led to cracking of the growing crystals due to high anisotropy of thermal expansion coefficients, which is characteristic of all double tungstates. It was found that 111 KHo(WO4)(2) single crystals were much bulkier than 110 ones, which had a rectangular cross-section elongated in the c direction. The bottom of 111 KHo(WO4)(2) crystals was built of one (111) plane, while in 110 crystals additional planes appeared that could not be eliminated by increasing the temperature gradient because such an increase led to cracking of crystals. As-grown 111 and 110 KHo(WO4)(2) single crystals were of good quality, free from such defects as bubbles, cracks, and inclusions. The morphology of KHo(WO4)(2) single crystals was investigated. Miller's indices of crystallographic faces (h k l) and edge indices [u v w] were found. Positions of the crystallographic axes a, b, and c in relation to growth faces and edges were found as well. The refractive indices and optical axes have been determined. The investigations of absorption spectra of KHo(WO4)(2) single crystals were performed in a wide spectral range (4000-40,000cm(-1)) at the temperature range 1.1 up to 300K. The trichroism of KHo(WO4)(2) was confirmed experimentally. This means that optical absorption is characterized by three different, independent spectra attributed to three main optical directions. (C) 2004 Elsevier B.V. All rights reserved.