The tellurophosphate K(4)P(8)Te(4): phase-change properties, exfoliation, photoluminescence in solution and nanospheres.

We describe the inorganic polymer K(4)P(8)Te(4) which is soluble, giving solutions that exhibit white emission upon 355 nm laser irradiation. An indirect band gap semiconductor (E(g) approximately 1.4 eV), K(4)P(8)Te(4) crystallizes in the space group P2(1)/m, with a = 6.946(1) A, b = 6.555(1) A, c = 9.955(2) A, and beta = 90.420(3) degrees at 173(2) K. The compound features infinite chains of [P(8)Te(4)(4-)] with covalent P-Te bonding and exhibits reversible crystal-glass phase-change behavior. When deposited from solution, the material forms highly crystalline K(4)P(8)Te(4) nanospheres. The thermal analysis, FT-IR, UV-vis diffuse reflectance, (31)P magic angle spinning solid-state NMR spectroscopy, and pair distribution function (PDF) analysis for the crystal and glass forms and ab initio electronic structure calculations by the screened-exchange local density function approximation are also reported. Speciation of K(4)P(8)Te(4) in solution studied with (31)P solution-state NMR spectroscopy, electrospray ionization mass spectrometry, and PDF analysis indicate exfoliation of [P(8)Te(4)(4-)] chains followed by rearrangement into molecular species.

Synthesis of a family of solids through the building-block approach: a case study with Ag(+) substitution in the ternary Na-Ge-Se system.

A new family of Ag-substituted pseudoquaternary alkali-seleno-germanates has been synthesized by two solid-state routes: the conventional flux method and metathesis. This family includes a series of semiconductors with varying amounts of Ag+ substituted for Na+ in Na8Ge4Se10 to form AgxNa(8-x)Ge4Se10, [x = 0.31 (I), 0.67 (II), 0.77 (III), 0.87 (IV), 1.05 (V), 1.09 (VI)] and another phase with a different composition AgxNa(6-x)Ge2Se7 (x = 1.76), VII, related to Na6Ge2Se7. In I-VI, Ge4Se10(8-) constitutes a 6-membered chairlike unit with a Ge-Ge bond, while in VII, a corner-shared dimer of GeSe4 tetrahedra (Ge2Se76-) acts as the building unit. The single-crystal structure analysis indicates that there is a phase transition from P to C2/c, in changing from pure Na8Ge4Se10 to AgxNa(8-x)Ge4 Se10 (I-VI), while there is no phase transition between pure Na6Ge2Se7 and AgxNa(6-x)Ge2Se7 (x = 1.76). The structures of I-VI may be described in terms of layers of cubic close-packed Se2- anions. In between the Se layers, octahedral holes fully occupied by Na+ and mixed Ag+/Na+ cations alternate with layers formed of octahedral holes fully occupied by Na+ and Ge26+ cations. Two adjacent Ge26+ cations form a chairlike Ge4Se10(8-) anion in which Ge-Ge bonds are oriented almost parallel to the Se layers. In contrast, VII does not have close-packed anions. Corner-shared GeSe4 tetrahedra (Ge2Se7(6-) dimer) and AgSe4 tetrahedra form layers that are cross-linked by Na/AgSe4 tetrahedra to form a 3-dimensional (3-D) structure. An optical property investigation indicates a red shift in the band gap of AgxNa(8-x)Ge4Se10 (x = 0.67)(II) as compared to that of pure Na8Ge4Se10. Raman data also indicate a red shift of the Ge-Se stretching mode in the Ag+-substituted phase II (x = 0.67) compared to that of Na8Ge4Se10.

[P6Se12]4-: a phosphorus-rich selenophosphate with low-valent P centers.

The new selenophosphate Rb4P6Se12 features the trans-decalin-like, [P6Se12]4- anion, a phosphorus-rich species that possesses three parallel P-P bonds and formally P2+ and P4+ centers. The synthesis of Rb4P6Se12 was accomplished with the reductive addition of P to RbPSe6 and represents an interesting example of how alkali chalcophosphates can serve as starting materials to produce new compounds under mild reaction conditions.