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Tetra­potassium cerium(IV) zirconium tetra­kis­(mono­phos­phate) crystallizes in the tetra­gonal system (space group I41/ amd). A complex disorder in K4CeZr(PO4)4 involves the mixing of Ce and Zr atoms on a single site with m2 symmetry and the splitting of P- and O-atom positions, equivalent to a rotation of the phosphate groups, to yield eight- and sixfold coordination environments around Ce and Zr, respectively. The K atoms are located in tunnels running parallel to the a and b axes. [ABSTRACT FROM AUTHOR]

Aliovalent KTP isomorphic compounds potassium chromium niobium oxide phosphate, KCr0.5Nb0.5OPO4, and potassium iron niobium oxide phosphate, KFe0.5Nb0.5OPO4, exhibit structures that differ from that of the non-centrosymmetric KTiOPO4. There are two crystallographically independent octa­hedral sites, M1 and M2, statistically occupied by Nb and Cr (or Fe) atoms. The M1O6 and M2O6 octa­hedra are connected alternately to form a chain with a cis– trans arrangement. The Nb atoms prefer the M2 sites arranged in a cis-like configuration. Each PO4 tetra­hedon has the P atom on a twofold axis. Site-splitting at the K-atom position is observed in both compounds. In the isomorphous structures, one Nb atom lies on an inversion centre and the other on a twofold axis. Similarly with the pairs of Fe/Cr sites, one is on an inversion centre and the other on a twofold axis. [ABSTRACT FROM AUTHOR]

Aliovalent K0.92In0.46Nb0.54OPO4 (KINP, potassium indium niobium oxide phosphate) features a chain structure involving corner-sharing [ MO6] octahedra ( M = In/Nb), which allows this compound to be recognized as a member of the KTiOPO4 (KTP) family. However, its crystallization in the polar space group P41 belonging to the 4/ m Laue group generates its own subclass of KTP-related compounds. The unit cell contains 18 symmetry-independent atoms occupying general positions. The cis– cis principle of octahedral arrangement is observed for each of the separate [ MO6]∞ chains. The observed variations in the cis– cis linkage are critically important for nonlinear optical properties and distinguish the present compound from other KTP-related structures. The anionic framework adopts one-dimensional tunnels running orthogonal to the ab plane. The K+ ions are arranged in the [001] direction at a distance of c. Merohedral twinning was detected during the structure refinement. [ABSTRACT FROM AUTHOR]