A series of new rare-earth borate crystals K7MRE2B15O30 (M = Zn, Cd, Pb; RE = Sc, Y, Gd, Lu) were synthesized by solid-state reaction method. All of the title compounds crystallize in the noncentrosymmetric trigonal space group R32, and their structures consist of the B5O10 groups and the KO8, KO6, MO6, and REO6 polyhedra. Each isolated B5O10 group is connected by the REO6 (Sc3+, Y3+, Gd3+, Lu3+) groups forming an intricate three-dimensional network, and the K+ and M2+ (Zn2+, Cd2+, Pb2+) cations fill into the void space. Because of the introduction of different nonlinear optical (NLO)-active structural units (cations with d10 electronic configuration and stereoactive lone pairs) into rare-earth metal borates, these compounds exhibit second harmonic generation (SHG) responses ranging from 1.5 to 2.1 times that of KH2PO4 (KDP), and are phase-matchable. Interestingly, this work indicates that the NLO properties can be artificially adjusted by systematically replacing the bivalent M2+ and trivalent RE3+ cations with NLO-active structural units, which provides a new way to design new NLO materials. Thermal analyses, IR spectra, and UV–Vis–NIR diffuse reflectance spectra were also reported in this work. In addition, the first-principles calculation was employed for better understanding of the structure–properties relationships of these compounds. Especially, the origins of SHG responses were well demonstrated by the SHG-density technique.

In the exploration of new infrared nonlinear optical (IR NLO) materials, element substitution has been developed as an effective way to adjust the structural features and material performances. A series of new IR NLO materials have been discovered in the I-Ba-M-Q system ( I = Li, Na, Cu, and Ag; M = Si, Ge, and Sn; Q = S and Se), and they undergo interesting structural transformation with different element substitution except Li analogues. Herein, we have successfully synthesized three selenides with different space groups (AgBaSiSe: I4̅2 m; AgBaGeSe and AgBaSnSe: I222) in the above system and studied their properties through experimental and theoretical methods. Remarkably, the detailed analysis on the structural changes and properties comparison was also systematically investigated in the I-Ba-M-Q system and the results indicate that the distortion degrees of different IQ tetrahedra play the critical role to cause the structural transformation with the M or Q elements substitution. More importantly, we have also found that the structural changes have the close relationship with the distance d( I- I) between adjacent I cations in the IBaSnSe system, which makes the four-membered rings formed by edge-sharing BaSe units change from the square to rhombus with the increase of d( I- I). The properties comparisons (band gap and NLO effect) in this system have been also systematically studied.

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PbMg(PO), PbZn(PO) and α-BaZn(PO) have been synthesized by the standard solid-state reaction and their structures were determined by single crystal X-ray diffraction for the first time. PbMg(PO), PbZn(PO) and α-BaZn(PO) crystallize in the monoclinic crystal system, P2/c, C2/c and the trigonal crystal system, P3[combining macron]m1, respectively. The structure of PbMg(PO) possesses a three-dimensional framework consisting of MgO octahedra and PO tetrahedra and the Pb cations reside in the [MgPO] tunnels. PbZn(PO) and α-BaZn(PO) are two-dimensional layered structures with [ZnPO] layers for PbZn(PO) and [ZnPO] honeycomb double layers for α-BaZn(PO). And in the structure of PbZn(PO), there are four different Zn-P-O layers, which is reported for the first time in zinc phosphates. In addition, thermal property analysis, and UV-Vis-NIR diffuse reflectance and infrared spectroscopy were also performed. First-principles theoretical studies were also conducted to aid the understanding of their band structures and densities of states.