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SrCd is Tetraauricupride structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Sr is bonded in a body-centered cubic geometry to eight equivalent Cd atoms. All Sr–Cd bond lengths are 3.53 Å. Cd is bonded in a body-centered cubic geometry to eight equivalent Sr atoms.
SrCd(PO3)4 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. Sr2+ is bonded in a 8-coordinate geometry to eight O2- atoms. There are a spread of Sr–O bond distances ranging from 2.51–2.81 Å. Cd2+ is bonded to six O2- atoms to form distorted CdO6 pentagonal pyramids that share corners with six PO4 tetrahedra. There are a spread of Cd–O bond distances ranging from 2.30–2.37 Å. There are four inequivalent P5+ sites. In the first P5+ site, P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share corners with two equivalent CdO6 pentagonal pyramids and corners with two PO4 tetrahedra. There are a spread of P–O bond distances ranging from 1.50–1.61 Å. In the second P5+ site, P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share a cornercorner with one CdO6 pentagonal pyramid and corners with two PO4 tetrahedra. There are a spread of P–O bond distances ranging from 1.50–1.62 Å. In the third P5+ site, P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share a cornercorner with one CdO6 pentagonal pyramid and corners with two PO4 tetrahedra. There are a spread of P–O bond distances ranging from 1.49–1.62 Å. In the fourth P5+ site, P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share corners with two equivalent CdO6 pentagonal pyramids and corners with two PO4 tetrahedra. There are a spread of P–O bond distances ranging from 1.51–1.60 Å. There are twelve inequivalent O2- sites. In the first O2- site, O2- is bonded in a bent 150 degrees geometry to two P5+ atoms. In the second O2- site, O2- is bonded in a bent 150 degrees geometry to two P5+ atoms. In the third O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Sr2+ and one P5+ atom. In the fourth O2- site, O2- is bonded in a 1-coordinate geometry to one Sr2+, one Cd2+, and one P5+ atom. In the fifth O2- site, O2- is bonded in a 1-coordinate geometry to one Sr2+, one Cd2+, and one P5+ atom. In the sixth O2- site, O2- is bonded in a bent 150 degrees geometry to two P5+ atoms. In the seventh O2- site, O2- is bonded in a distorted single-bond geometry to one Sr2+, one Cd2+, and one P5+ atom. In the eighth O2- site, O2- is bonded in a bent 120 degrees geometry to two P5+ atoms. In the ninth O2- site, O2- is bonded in a 1-coordinate geometry to one Sr2+, one Cd2+, and one P5+ atom. In the tenth O2- site, O2- is bonded in a distorted single-bond geometry to one Sr2+, one Cd2+, and one P5+ atom. In the eleventh O2- site, O2- is bonded in a distorted single-bond geometry to one Sr2+ and one P5+ atom. In the twelfth O2- site, O2- is bonded in a distorted single-bond geometry to one Sr2+, one Cd2+, and one P5+ atom.
The AM 2 Pn 2 (A= Ca, Sr, Ba, Yb, Mg; M = Zn, Cd, Mg; and Pn = N, P, As, Sb, Bi) family of Zintl phases has been known as thermoelectric materials and has recently gained much attention for highly promising materials for solar absorbers in single-junction and tandem solar cells. In this paper, we will, from first principles, explore the entire family of AM 2 Pn 2 compounds in terms of their ground-state structure, thermodynamic stability, and electronic structure. We also perform photoluminescence spectroscopy on bulk powder and thin film samples to verify our results, including the first measurements of the band gaps of SrCd 2 P 2 and CaCd 2 P 2 . The AM 2 Pn 2 compounds exhibit broad stability, are mostly isostructural to CaAl 2 Si 2 (P$\overline{3}$m1), and cover a wide range of band gaps from 0 to beyond 3 eV. This could make them useful for a variety of purposes, for which we propose several candidates, such as CaZn 2 N 2 for tandem top cell solar absorbers and SrCd 2 Sb 2 and CaZn 2 Sb 2 for infrared detectors. By examining the band structures of the AM 2 Pn 2 , we find that Mg 3 Sb 2 has the most promise as a thermoelectric material due to several off-Γ valence band pockets, which are unique to it among the compositions studied here.
Recent studies have predicted that magnetic EuCd 2 As 2 can host several different topological states depending on its magnetic order, including a single pair of Weyl points. Here we report on the bulk properties and band inversion induced by pressure in the nonmagnetic analogs AECd 2 As 2 (AE = Ca, Sr, Ba) as studied with density functional theory calculations. Under ambient pressure we find that these compounds are narrow band gap semiconductors, in agreement with experiment. In this work, the size of the band gap is dictated by both the increasing ionicity across the AE series which tends to increase the band gap, as well as the larger nearest neighbor Cd-As distance from increasing atomic size which can decrease the band gap because the conduction band edge is an antibonding state derived mostly from Cd 5s orbitals. The combination of these two competing effects results in a nonmonotonic change of the band gap size across the AE series with SrCd 2 As 2 having the smallest band gap among the three compounds. The application of negative pressure reduces this band gap and causes the band inversion between the Cd 5s and As 4p orbitals along the Γ-A direction to induce a pair of Dirac points. The topological nature of the Dirac points is then confirmed by finding the closed Fermi arcs on the ($10\bar{1}0$) surface.