Band Gap Tuning in New Strontium Seleno-Stannates

Abstract

The new ternary seleno-stannates Sr2SnSe5 and SrSn2Se4 were both prepared from directly reacting the elements in stoichiometric ratios at 850 °C, followed by slow cooling. Sr2SnSe5 crystallizes in the Eu2SnSe5 type with SnIV, Se-II, and nonlinear Se3 2- units, space group P21212, with lattice dimensions of a = 1205.7(2) pm, b = 1658.3(3) pm, c = 863.9(2) pm, V = 1.7272(6) nm3 (Z = 8). SrSn2Se4 forms a distorted variant of the SrIn2Se4 type with mixed valences, i.e., SnII and SnIV instead of two InIII. The distortion is reflected in lowering the symmetry from Fddd to Fdd2, occurring with differently sized and distorted SnIISe4 and SnIVSe4 tetrahedra. The lattice dimensions are a = 2189.2(1) pm, b = 2189.3(1) pm, c = 1354.70(7) pm, V = 6.4928(6) nm3 (Z = 32). The SnII atoms may be replaced by Mg, producing the new isostructural compound SrMgSnSe4 with a = 2176.0(1) pm, b = 2185.9(1) pm, c = 1340.60(9) pm, V = 6.3765(7) nm3. SrMgSnSe4 is a red material (indicative of a band gap of 2.0 eV), while the two other seleno-stannates are black, pointing toward gaps <1.7 eV. The computed band gaps are 1.8 eV for SrMgSnSe4, 0.9 eV for Sr2SnSe5, and 0.2 eV for SrSn2Se4.

Barium manganese(II) selenostannate(IV), BaMnSnSe4

“…The Mn-Se and Sn-Se bonds vary from 2.50 Å to 2.61 Å and 2.49 Å to 2.54 Å, respectively. Similar distortions were observed in the case of the mixed valent selenostannate SrSn 2 Se 4 , in SrMgSnSe 4 , (Assoud et al, 2004) and in BaMSnS 4 (M = Mn, Zn, Cd, Hg) (Teske, 1980a,b,c,d), with Se-Sn-Se bond angles ranging from 94° to 129°. The tetrahedra in BaMnSnSe 4 are interconnected by sharing edges between the MnSe 4 and SnSe 4 units.…”

Barium manganese(II) selenostannate(IV), BaMnSnSe4

“…The Mn-Se and Sn-Se bonds vary from 2.50 Å to 2.61 Å and 2.49 Å to 2.54 Å, respectively. Similar distortions were observed in the case of the mixed valent selenostannate SrSn 2 Se 4 , in SrMgSnSe 4 , (Assoud et al, 2004) and in BaMSnS 4 (M = Mn, Zn, Cd, Hg) (Teske, 1980a,b,c,d), with Se-Sn-Se bond angles ranging from 94° to 129°. The tetrahedra in BaMnSnSe 4 are interconnected by sharing edges between the MnSe 4 and SnSe 4 units.…”

Synthesis, Structure, and Thermoelectric Properties of Barium Copper Polychalcogenides with Chalcogen‐Centered Cu Clusters and Te22– Dumbbells

“…This is in agreement with the charge-balanced Ba 2+ (Cu + ) 6 Se 2--(Te 2 2-) 3 formula, derived from the above-mentioned formal charges. Analogous observations, namely that the bottom of the conduction band is composed of (empty) antibonding states originating from bonds between the chalcogen atoms, were also made for Sr 2 SnSe 5 , [34] Ba 2 SnTe 5 , [35] and Ba 7 Cu 2 -Te 14 . [13] It is concluded that the presence of polychalcogenide units favors narrower band gaps, which is advantageous for the thermoelectric energy conversion.…”

Electronic Structure and Physical Properties of the Semiconducting Polytelluride Ba2SnTe5 with a Unique Te54- Unit

“…The former is reflected in (mostly) bonding, and the latter two are reflected in antibonding Sn−Te interactions (right part of Figure ). We conclude that the band gap of 0.25 eV occurs between filled Te p and empty Sn s states, a significant difference from Sr 2 SnSe 5 , where the gap separates filled Se p states from empty Se−Se antibonding contributions . This difference arises from the energetically higher lying Te states, as compared to the Se states, which renders them closer in energy to the Sn s states.…”

Barium manganese(II) selenostannate(IV), BaMnSnSe4

“…The Mn-Se and Sn-Se bonds vary from 2.50 Å to 2.61 Å and 2.49 Å to 2.54 Å, respectively. Similar distortions were observed in the case of the mixed valent selenostannate SrSn 2 Se 4 , in SrMgSnSe 4 , (Assoud et al, 2004) and in BaMSnS 4 (M = Mn, Zn, Cd, Hg) (Teske, 1980a,b,c,d), with Se-Sn-Se bond angles ranging from 94° to 129°. The tetrahedra in BaMnSnSe 4 are interconnected by sharing edges between the MnSe 4 and SnSe 4 units.…”

Synthesis, Structure, and Thermoelectric Properties of Barium Copper Polychalcogenides with Chalcogen‐Centered Cu Clusters and Te22– Dumbbells

“…This is in agreement with the charge-balanced Ba 2+ (Cu + ) 6 Se 2--(Te 2 2-) 3 formula, derived from the above-mentioned formal charges. Analogous observations, namely that the bottom of the conduction band is composed of (empty) antibonding states originating from bonds between the chalcogen atoms, were also made for Sr 2 SnSe 5 , [34] Ba 2 SnTe 5 , [35] and Ba 7 Cu 2 -Te 14 . [13] It is concluded that the presence of polychalcogenide units favors narrower band gaps, which is advantageous for the thermoelectric energy conversion.…”

Electronic Structure and Physical Properties of the Semiconducting Polytelluride Ba2SnTe5 with a Unique Te54- Unit

“…The former is reflected in (mostly) bonding, and the latter two are reflected in antibonding Sn−Te interactions (right part of Figure ). We conclude that the band gap of 0.25 eV occurs between filled Te p and empty Sn s states, a significant difference from Sr 2 SnSe 5 , where the gap separates filled Se p states from empty Se−Se antibonding contributions . This difference arises from the energetically higher lying Te states, as compared to the Se states, which renders them closer in energy to the Sn s states.…”

Barium manganese(II) selenostannate(IV), BaMnSnSe4

“…The Mn-Se and Sn-Se bonds vary from 2.50 Å to 2.61 Å and 2.49 Å to 2.54 Å, respectively. Similar distortions were observed in the case of the mixed valent selenostannate SrSn 2 Se 4 , in SrMgSnSe 4 , (Assoud et al, 2004) and in BaMSnS 4 (M = Mn, Zn, Cd, Hg) (Teske, 1980a,b,c,d), with Se-Sn-Se bond angles ranging from 94° to 129°. The tetrahedra in BaMnSnSe 4 are interconnected by sharing edges between the MnSe 4 and SnSe 4 units.…”

Synthesis, Structure, and Thermoelectric Properties of Barium Copper Polychalcogenides with Chalcogen‐Centered Cu Clusters and Te22– Dumbbells

“…This is in agreement with the charge-balanced Ba 2+ (Cu + ) 6 Se 2--(Te 2 2-) 3 formula, derived from the above-mentioned formal charges. Analogous observations, namely that the bottom of the conduction band is composed of (empty) antibonding states originating from bonds between the chalcogen atoms, were also made for Sr 2 SnSe 5 , [34] Ba 2 SnTe 5 , [35] and Ba 7 Cu 2 -Te 14 . [13] It is concluded that the presence of polychalcogenide units favors narrower band gaps, which is advantageous for the thermoelectric energy conversion.…”

Electronic Structure and Physical Properties of the Semiconducting Polytelluride Ba2SnTe5 with a Unique Te54- Unit

“…The former is reflected in (mostly) bonding, and the latter two are reflected in antibonding Sn−Te interactions (right part of Figure ). We conclude that the band gap of 0.25 eV occurs between filled Te p and empty Sn s states, a significant difference from Sr 2 SnSe 5 , where the gap separates filled Se p states from empty Se−Se antibonding contributions . This difference arises from the energetically higher lying Te states, as compared to the Se states, which renders them closer in energy to the Sn s states.…”