Strong anisotropic thermal expansion in borates

Abstract

The paper summarizes results of high‐temperature single crystal X‐ray diffraction studies for six borate structures and high‐temperature powder X‐ray diffraction data for about 50 borates. The characteristics on thermal expansion of 50 borates of univalent, bivalent and trivalent metals of different dimensionality and anion structure are listed. The basic principles of high‐temperature borate crystal chemistry emerge from these studies and the regularities of borate thermal behavior are revealed. On heating the BO3 and BO4 polyhedra and rigid groups consisting of these polyhedra practically do not change their configuration and size but sometimes rotate like hinges exhibiting greatly anisotropic thermal expansion including linear negative expansion. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Borates—Crystal Structures of Prospective Nonlinear Optical Materials: High Anisotropy of the Thermal Expansion Caused by Anharmonic Atomic Vibrations

“…These results have been summarized in [26,34,42,43] and are briefly described here. In this respect, the rigidity of a group means that the group cannot change its configuration as well as its size with varying temperature.…”

“…For most borates a high anisotropy of the thermal expansion is characteristic, as has been found in [26,34,42]. It can be analysed to serve as an additional criterion for NLO borates, like high values of ng − np birefringence [12,15], etc.…”

Near‐Zero Thermal Expansion and High Ultraviolet Transparency in a Borate Crystal of Zn4B6O13

“…For instance, β‐BaB 2 O 4 (BBO) and LiB 3 O 5 (LBO) are widely used as the key devices for UV laser generation . More importantly, borate crystals are suitable for ZTE exploration because their structures contain the typical features for anomalous thermal expansion . A boron atom can be threefold or fourfold coordinated with oxygen atoms to form the [BO 3 ] triangle or the [BO 4 ] tetrahedron.…”

“…These microscopic groups are strongly covalent and the bond lengths and angles remain almost constant as the ambient temperature varies . Further interconnections of the rigid B–O groups can construct 1D, 2D, or 3D frameworks with a large structural diversity, in which rotation between the B–O groups may result in the anomalous thermal expansion . Many borates, such as LBO, KBe 2 BO 3 F 2 , LiBeBO 3 , KZnB 3 O 6 , and CsSiB 2 O 6 exhibit the anomalous‐thermal‐expansion (negative‐thermal‐expansion (NTE)) effect.…”

Thermal “order‐disorder” behaviour in (Na1‐xKx)4B8O14 solid solutions investigated by X‐ray powder diffraction

“…This is in accordance with fundamental principles of high temperature borate crystal chemistry [15]. Volumetric thermal expansion of the structure is caused by an increase in cation-oxygen polyhedra volume accompanied by a change of the angles between rigid boron-oxygen groups.…”