M₂Cd₃B₁₆O₂₈ (M = Rb, Cs): Two Isostructural Alkali Cadmium Borates with a New Type of Borate Layer

Abstract

Two new isostructural compounds, M2Cd3B16O28 (M = Rb, Cs), have been synthesized by a high‐temperature solid‐state reaction for the first time. M2Cd3B16O28 (M = Rb, Cs) crystallize in the monoclinic space group C2/c, with a = 21.4856(10) Å, b = 7.6852(4) Å, c = 13.3694(8) Å, β = 91.705(3)°, Z = 4 for Rb2Cd3B16O28 and a = 21.621(3) Å, b = 7.6820(11) Å, c = 13.4170(18) Å, β = 91.068(8)°, Z = 4 for Cs2Cd3B16O28. The compounds crystallize with (B16O34)∞ double layers bridged by Cd2+ cations to form three dimensional (3D) structures. The M+ (M = Rb, Cs) cations reside in the tunnels of the (B16O34)∞ double layers. First‐principle electronic structure calculations show that the calculated band gaps of M2Cd3B16O28 (M = Rb, Cs) are 4.67 and 4.71 eV, respectively, which are in good agreement with the experimental values estimated from UV/Vis/NIR absorption spectroscopy by using the Kubelka–Munk equation, and the observed absorption peak is assigned as a charge transfer from the O 2p state to the Cd 5s state.

Crystal chemistry of halurgite, Mg4[B8O13(OH)2]2·7H2O, a microporous heterophylloborate mineral

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Fig. 4.The crystal structures of two heterophylloborates: ( a , b ) halurgite, along the b and c axes, respectively (positions of O atoms of OH groups are shown as small green circles, O atoms of H 2 O molecules are given as blue circles); and ( c , d ) synthetic M 2 Cd 3 B 16 O 28 ( M = Rb or Cs) (drawn after Dong et al ., 2013), along the b and c axes, respectively. The unit cells are outlined.

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“…A similar FBB was reported in the structures of four synthetic microporous borates: M 2 Cd 3 B 16 O 28 ( M = Rb or Cs) (Dong et al ., 2013) and M 2 Ca 3 B 16 O 28 ( M = Rb or Cs) (Zhang et al ., 2016) in which the [B 8 O 18 ] FBBs are connected to form [B 8 O 15 ] ∞ layers very similar to those found in halurgite. However, unlike the latter, in these synthetic H-free compounds two face-to-face [B 8 O 15 ] ∞ layers are further bridged by corner-sharing O atoms to build a 3D open framework [B 8 O 14 ] ∞ (Figs 4 c , d ), whereas in halurgite, these sites are occupied by O atoms of OH groups and do not form B–O–B bridges.…”

“…All Mg cations are octahedrally coordinated in the mineral, however, different Mg octahedra play different roles in the structure: Mg(1–2)O 6 octahedra are isolated from each other and bridge the borate layers from the non ‘face-to-face’ sides sharing all vertices with B polyhedra whereas Mg(3)(OH) 2 (H 2 O) 4 octahedra link the layers from the ‘face-to-face’ side (Figs 4 a , b ). The arrangement of Mg(1–2)O 6 octahedra is very close to those found for CdO 6 octahedra in M 2 Cd 3 B 16 O 28 ( M = Rb or Cs) (Dong et al ., 2013) and Ca polyhedra (which could be reduced to octahedra if elongated Ca–O distances are omitted) in M 2 Ca 3 B 16 O 28 ( M = Rb or Cs) (Zhang et al ., 2016), as shown in Figs 4 c , d .…”

“…In the structures of synthetic M 2 Cd 3 B 16 O 28 and M 2 Ca 3 B 16 O 28 ( M = Rb or Cs) (Dong et al ., 2013; Zhang et al ., 2016) large M cations occupy tunnels in the borate double layers (Figs 4 c , d ). In halurgite Mg cations and H 2 O molecules are located between the single borate layers (Figs 4 a , b ), which could be considered as a half of the double layer in M 2 Cd 3 B 16 O 28 and M 2 Ca 3 B 16 O 28 ( M = Rb or Cs).…”

Synthesis, characterization, and theoretical analysis of three new nonlinear optical materials K7MRE2B15O30 (M= Ca and Ba, RE= La and Bi)

“…Numbers of famous NLO crystals have been obtained, including β-BaB 2 O 4 (β-BBO) [8], LiB 3 O 5 (LBO) [9], CsB 3 O 5 (CBO) [10], CsLiB 6 O 10 (CLBO) [11], KBe 2 BO 3 F 2 (KBBF) [12], Sr 2 Be 2 B 2 O 7 [13], K 2 Al 2 B 2 O 7 (KAB) [14], KTiOPO 4 (KTP) [15], KH 2 PO 4 (KDP) [16], AgGaQ 2 (Q = S, Se) [17,18], and ZnGeP 2 (ZGP) [19], which can expand the spectral ranges of solid state lasers from deep-ultraviolet (DUV) to infrared (IR). In the UV and DUV range, borates play a dominant role for their various structural features [20][21][22][23][24], wide transparency window, and large secondharmonic generation (SHG) response [25][26][27][28][29][30]. For example, β-BBO exhibits large SHG response about 6× KDP and its UV cutoff edge is at about 189 nm; LBO and KBBF not only possess large SHG response about 3× and 1.2× KDP, but also show short UV cutoff edge at about 160 and 155 nm, respectively.…”