Observation and enhancement through alkali metal doping of p-type conductivity in the layered oxyselenides Sr2ZnO2Cu2Se2 and Ba2Zn1−xO2−xCu2Se2†

The optoelectronic properties of two layered copper oxyselenide compounds, with nominal composition Sr₂ZnO₂Cu₂Se₂ and Ba₂ZnO₂Cu₂Se₂, have been investigated to determine their suitability as p-type conductors. The structure, band gaps and electrical conductivity of pristine and alkali-metal-doped samples have been determined. We find that the strontium-containing compound, Sr₂ZnO₂Cu₂Se₂, adopts the expected tetragonal Sr₂Mn₃SbO₂ structure with I4/mmm symmetry, and has a band gap of 2.16 eV, and a room temperature conductivity of 4.8 × 10⁻¹ S cm⁻¹. The conductivity of the compound could be increased to 4.2 S cm⁻¹ when sodium doped to a nominal composition of Na_0.1Sr_1.9ZnO₂Cu₂Se₂. In contrast, the barium containing material was found to have a small zinc oxide deficiency, with a sample dependent compositional range of Ba₂Zn_1−xO_2−xCu₂Se₂ where 0.01 < x < 0.06, as determined by single crystal X-ray diffraction and powder neutron diffraction. The barium-containing structure could also be modelled using the tetragonal I4/mmm structure, but significant elongation of the oxygen displacement ellipsoid along the Zn–O bonds in the average structure was observed. This indicated that the oxide ion position was better modelled as a disordered split site with a displacement to change the local zinc coordination from square planar to linear. Electron diffraction data confirmed that the oxide site in Ba₂Zn_1−xO_2−xCu₂Se₂ does not adopt a long range ordered arrangement, but also that the idealised I4/mmm structure with an unsplit oxide site was not consistent with the extra reflections observed in the electron diffractograms. The band gap and conductivity of Ba₂Zn_1−xO_2−xCu₂Se₂ were determined to be 2.22 eV and 2.0 × 10⁻³ S cm⁻¹ respectively. The conductivity could be increased to 1.5 × 10⁻¹ S cm⁻¹ with potassium doping in K_0.1Ba_1.9Zn_1−xO_2−xCu₂Se₂. Hall measurements confirmed that both materials were p-type conductors with holes as the dominant charge carriers.