Syntheses, Structures, and Spectroscopic Properties of K₉Nd[PS₄]₄, K₃Nd[PS₄]₂, Cs₃Nd[PS₄]₂, and K₃Nd₃[PS₄]₄

Four new quaternary alkali neodymium thiophosphates K9Nd[PS4]4 (1), K3Nd[PS4]2 (2), Cs3Nd[PS4]2 (3), and K3Nd3[PS4]4 (4) were synthesized by reacting Nd with in situ formed fluxes of K2S3 or Cs2S3, P2S5 and S in appropriate molar ratios at 973 K. Their crystal structures are determined by single crystal X-ray diffraction. Crystal data: 1: space group C2/c, a = 20.1894(16), b = 9.7679(5), c = 17.4930(15) Å, β = 115.66(1)°, and Z = 4; 2: space group P21/c, a = 9.1799(7), b = 16.8797(12), c = 9.4828(7) Å, β = 90.20(1)°, and Z = 4; 3: space group P21/n, a = 15.3641(13), b = 6.8865(4), c = 15.3902(13) Å, β = 99.19(1)°, and Z = 4; 4: space group C2/c, a = 16.1496(14), b = 11.6357(7), c = 14.6784(11) Å, β = 90.40(1)°, and Z = 4. The structure of 1 is composed of one-dimensional 1∞{Nd[PS4]4}9− chains and charge balancing K+ ions. Within the chains, eight-coordinated Nd3+ ions, which are mixed with K+ ions, are connected by [PS4]3− tetrahedra. The crystal structures of 2 and 3 are characterized by anionic chains 1∞{Nd[PS4]2}3− being separated by K+ or Cs+ ions. Along each chain the Nd3+ ions are bridged by [PS4]3− anions. The difference between the structures of 2 and 3 is that in 2 the Nd3+ ions are coordinated by four edge-sharing [PS4]3− tetrahedra while in 3 each Nd3+ ion is surrounded by one corner-sharing, one face-sharing, and two edge-sharing [PS4]3− tetrahedra. The structure of 4 is a three-dimensional network with K+ cations residing in tunnels running along [110] and [1̅10]. The {Nd(1)S8} polyhedra share common edges with four [PS4] tetrahedra forming one-dimensional chains 1∞{Nd[PS4]2}3− running along [110] and [1̅10]. The chains are linked by {Nd(2)S8} polyhedra yielding the final three-dimensional network 3∞{Nd[PS4]2}3−. The internal vibrations of both crystallographically independent [PS4]3− anions of 2−4 have been assigned in the range 200−650 cm−1 by comparison of their corresponding far/mid infrared and Raman spectra (λexc = 488 nm) on account of locally imposed C1 symmetry. In the Fourier-transform-Raman spectrum (λexc = 1064 nm) of 2−4, very similar well-resolved electronic Raman (ER) transitions from the electronic Nd3+ ground-state to two levels of the 4I9/2 ground manifold and to the six levels of the 4I11/2 manifold have been determined. Resonant Raman excitation via a B-term mechanism involving the 4I15/2 and 4F3/2 intermediate states may account for the significant intensity enhancement of the ER transitions with respect to the symmetric P−S stretching vibration ν1. Broad absorptions in the UV/vis/NIR diffuse reflectance spectrum at 293 K in the range 5000−25000 cm−1 of 2−4 are attributed to spin-allowed excited quartet states [4(I < F < S < G < D)] and spin-forbidden doublet states [2(H < G < K < D < P)] of Nd3+. A luminescense spectrum of 3 obtained at 15 K by excitation with 454.5 nm shows multiplets of narrow lines that reproduce the Nd3+ absorptions. Sharp and intense luminescence lines are produced instead by excitation with 514.5 nm. Lines at 18681 (4G7/2), 16692 (4G5/2), 14489 (4F9/2), and 13186 cm−1 (4F7/2) coincide with the corresponding absorptions. Hypersensitive 4G5/2 is split by 42 cm−1. The most intense multiplet at about 16500 cm−1 is assigned to the transition from 4G5/2 to the Stark levels of the ground manifold 4I9/2.