α-Ca₂CdP₂ and β-Ca₂CdP₂: Two Polymorphic Phosphide-Based Infrared Nonlinear Crystals with Distorted NLO-Active Tetrahedral Motifs Realizing Large Second Harmonic Generation Effects and Suitable Band Gaps

Abstract

Two polymorphic phosphide-based infrared (IR) nonlinear optical (NLO) crystals, α-Ca2CdP2 and β-Ca2CdP2, were obtained by combining alkaline-earth metals and d10 transition metals using metal flux and metal salt flux methods, respectively. The crystal structure of α-Ca2CdP2 is orthorhombic in the space group Cmc21 (no. 36), while the structure of β-Ca2CdP2 is monoclinic in the space group Cm (no. 8). Both are two-dimensional layered structures that are composed of CdP4 tetrahedra layers via sharing vertices, which stack along the b-axis and the c-axis, respectively. The second harmonic generation (SHG) measurements manifest that α-Ca2CdP2 and β-Ca2CdP2 exhibit strong SHG intensities (1.41 and 3.28× that of AgGaS2 at a 2050 nm laser, respectively). Other optical measurements indicate that α-Ca2CdP2 and β-Ca2CdP2 have suitable band gaps (1.98 and 1.55 eV, respectively), high laser-induced damage thresholds (4.5 and 3.1× that of AgGaS2 at 1064 nm laser, respectively) and appropriate birefringence (0.12 and 0.20 at 2050 nm, respectively) in addition to covering wide infrared transparent regions. The research on α-Ca2CdP2 and β-Ca2CdP2 demonstrates that they are potential IR NLO candidates. Theoretical calculations uncover that their SHG effects are from distorted CdP4 tetrahedra, highlighting that tetrahedral motifs, including d10 transition metals, would be ideal NLO-active building blocks.

Structural Complexity and Tuned Thermoelectric Properties of a Polymorph of the Zintl Phase Ca2CdSb2 with a Non-centrosymmetric Monoclinic Structure

“…A structurally related monoclinic modification was recently reported for β-Ca 2 CdAs 2 and β-Ca 2 CdP 2 phases. 10,16 As reported herein, Ca 2 CdSb 2 (-m) polymorph also crystallizes in the monoclinic crystal system, yet with a noticeably larger unit cell and much more complex structure. This structure is with the non-centrosymmetric space group Cm (No.…”

“…AE 2 Cd Pn 2 ( AE = Ca, Sr., Ba, Eu, Yb; Pn = P, As, Sb) ternary Zintl phases are customarily known to crystallize with two possible structuresthe centrosymmetric space group Pnma and the non-centrosymmetric space group Cmc 2 1 . ,,, Polymorphism is also known for this class of materials. A structurally related monoclinic modification was recently reported for β-Ca 2 CdAs 2 and β-Ca 2 CdP 2 phases. , As reported herein, Ca 2 CdSb 2 (-m) polymorph also crystallizes in the monoclinic crystal system, yet with a noticeably larger unit cell and much more complex structure. This structure is with the non-centrosymmetric space group Cm (No.…”

“…However, another key twist to the storyline of the "2−1−2" phases is their propensity to form polymorphs/polytypes as found in Ca 2 CdP 2 and Ca 2 CdAs 2 for example. 10,16,17 Often times, the formation of the different forms appears to be in correlation with the choice of metal flux. 1 Structurally and compositionally related "9−4−9" phases are also amenable to polymorphism, as was recently reported for the Ca 9 (Zn,In) 4 Sb 9 phase.…”

Hg2(SeO3)(SO4): the first sulfate selenite with large birefringence explored from d10transition metal compounds

“…The d 10 -TMs are beneficial for improving the birefringence of crystals due to their high polarizability and deformability. Many new compounds containing d 10 cations, like Hg 2+ and Cd 2+ , with a large birefringence have been reported, such as AgHgPS 4 (0.11 at 2090 nm), 27 LiHgPO 4 (0.068 at 1064 nm), 28 b-Ca 2 CdP 2 (0.2 at 2050 nm), 29 etc. [30][31][32][33][34][35] To explore new metal sulfate selenites with large birefringence, d 10 -TMs were chosen as the charge-balance cations.…”

(C4H10NO)PbX3 (X = Cl, Br): Design of Two Lead Halide Perovskite Crystals with Moderate Nonlinear Optical Properties

“…The second-order nonlinear optical (NLO) crystals that can broaden output wavelengths play a significant function in modern optical techniques, such as precise micromanufacturing, electro-optical amplitude modulation, photoetching, biological tissue imaging, and optical data storage. − The majority of the commercial NLO materials are inorganic crystals, such as KBe 2 BO 3 F 2 (KBBF), β-BaB 2 O 4 (β-BBO), LiB 3 O 5 (LBO), KH 2 PO 4 (KDP), and AgGaS 2 (AGS), etc. However, because of the enormous cost and time-consuming preparation of some NLO inorganic crystals, they cannot fill the fast-growing requirements of laser techniques, and continuous endeavors have been made to design and seek novel crystals with noncentrosymmetric (NCS) structuresa fundamental requirement for the SHG effect for the development of fundamental research as well as practical applications. − …”