Helical Polymer ¹/_∞[P₂Se₆^2-]:  Strong Second Harmonic Generation Response and Phase-Change Properties of Its K and Rb Salts

Abstract

The selenophosphates A2P2Se6 (A = K, Rb) crystallize in the chiral trigonal space group P3121, with a = 7.2728(9) A, c = 18.872(4) A, and Z = 3 at 298(2) K and a = 14.4916(7) A, c = 18.7999(17) A, and Z = 12 at 173(2) K for K+ salt and a = 7.2982(5) A, c = 19.0019(16) A, and Z = 3 at 100(2) K for Rb+ salt. The A2P2Se6 feature parallel one-dimensional helical chains of 1/infinity[P2Se62-] which depict an oxidative polymerization of the ethane-like [P2Se6]4- anion. On cooling well below room temperature K2P2Se6 exhibits a displacive phase transition to a crystallographic subgroup and forms a superstructure with a cell doubling along the a- and b-axes. The Rb analogue does not exhibit the phase transition. The compounds are air stable and show reversible glass-crystal phase-change behavior with a band gap red shift of 0.11 and 0.22 eV for K+ and Rb+ salts, respectively. Raman spectroscopy, 31P magic angle spinning solid-state NMR, and pair distribution function (PDF) analysis for crystalline and glassy K2P2Se6 give further understanding of the phase transition and the local structure of the amorphous state. K2P2Se6 exhibits excellent mid-IR transparency and a strong second harmonic generation (SHG) response. The SHG response is type-I phase-matchable and in the wavelength range of 1000-2000 nm was measured to be 50 times larger than that of the commercially used material AgGaSe2. Glassy K2P2Se6 also exhibits an SHG response without the application of electric field poling. In connection with the NLO properties the thermal expansion coefficients for K2P2Se6 are reported.

Kinetics of Nonlinear Optical Response at Insulator–Metal Transition in Vanadium Dioxide

“…where ( ) 1 α ω is the linear absorption coeffi cient, ( ) 3 α ω is the third-order nonlinear absorption coeffi cient, and I is the intensity of incident light. The linear absorption coeffi cient can be expressed as [ 42 ] Adv.…”

“…For saturable absorption, the sign of ( ) 3 α ω is negative and Equation ( 3) can be rearranged as [ 32,43 ] α ω are the saturable and nonsaturable absorption components, respectively, and sat I is the saturation intensity, defi ned as the optical intensity required in a steady state to reduce the absorption to a half of its unbleached value. For saturable absorption, the sign of ( ) 3 α ω is negative and Equation ( 3) can be rearranged as [ 32,43 ] α ω are the saturable and nonsaturable absorption components, respectively, and sat I is the saturation intensity, defi ned as the optical intensity required in a steady state to reduce the absorption to a half of its unbleached value.…”

“…Details of the calculation are shown in the Methods Section. The fi nal tetragonal state is also displayed in Figure 1 d. The theoretical calculation indicates that no matter whether in the insulating or metallic states, VO 2 generates a Phase transitions are ubiquitous in condensed-matter systems, and the investigation of their kinetics is known to be challenging, [1][2][3][4] but the signifi cant discoveries, such as superconductive phase transitions, [ 5 ] ferroelectric and antiferroelectric phase transitions, [ 6 ] amorphous and crystalline transitions, [ 7 ] insulator-metal transitions (IMT), [8][9][10][11][12][13] etc., have produced a great deal of interest in the physics community. [ 13 ] As the IMT progresses, the VO 2 band gap decreases and its phase enters a tetragonal metallic state from the monoclinic insulating state, as shown in Figure 1 b,c.…”

Predicting Global Minimum in Complex Beryllium Borate System for Deep-ultraviolet Functional Optical Applications

“…However, both α-BBO and KBBF do not have DUV commercial availability since α-BBO exhibits high UV cutoff edge (189 nm) and KBBF suffers from a strong layering tendency. As such, it is the subject of considerable interest to investigate DUV birefringent and NLO materials1819202122. In the past few years, first principle calculations have been an efficient method to evaluate and verify the physical and chemical properties of optical materials232425.…”

Semiconducting Properties and Phase-Matching Nonlinear Optical Response of the One-Dimensional Selenophosphates ANb2PSe10 (A = K, Rb, and Cs)

“…2,5,9,10 These materials present a wide array of structural features ranging from molecular units to 3-D networks 3 ]. 3,6,11,12 Despite the extensive research done on alkali metal late transition metal chalcophosphates, few reports exist on such compounds with early transition metals, especially selenium.…”