A discussion on `Domain formation and phase transitions in the wurtzite-based heterovalent ternaries: a Landau theory analysis'.

Joachim Breternitz,Paul C Quayle

doi:10.1107/s2053273321001376

Abstract

Heterovalent ternary nitrides are considered one of the promising classes of materials for photovoltaics, combining attractive physical properties with low toxicity and element abundance. One of the front-runner systems under consideration is ZnSnN2. Although it is nominally a ternary compound, no clear crystallographic evidence for cation ordering has been observed so far. An attempt to elucidate this discrepancy [Quayle (2020). Acta Cryst. A76, 410-420] was the trigger for an intensive discussion between the authors, and an agreement was reached to elaborate on some points in order to set things in perspective. Rather than using a conventional comment-answer scheme, this is published in the form of a joint discussion to celebrate constructive criticism and collegiality.

Highlights

Binary III–V nitrides in the wurtzite-type structure are candidate materials for photovoltaic applications (Jani et al, 2007); indium and gallium are rather scarce and expensive elements, and more earth-abundant alternatives are sought-after for low-cost applications
The II–IV–N2 compounds are a promising class of materials which are formally derived from III–V materials by replacing the trivalent cation in the binary compound by a stoichiometric mixture of divalent and tetravalent cations
Density functional theory (DFT) calculations suggest that the bandgap of ZnSnN2 can be lowered by 0.85 eV in fully disordered material, with a randomized cation sublattice, compared with perfectly ordered material with the -NaFeO2-type structure (Makin et al, 2019)

Summary

How it all started

After the publication of Quayle (2020), I received a message titled ‘On: ‘Domain formation and phase transitions in the wurtzite-based heterovalent ternaries: a Landau theory analysis’ in which Joachim Breternitz pointed out, in the most courteous and professional manner, an error in the proposed Landau theory. What followed was an exchange concerning the nature of atomic disorder in the wurtzite-based ternaries; an exchange that has led us to a satisfying impasse. As discussed later, Quayle (2020) treats two different crystal structures, both members of the same space-group type but with different unit-cell sizes and atom locations, as one and the same, and the Landau theory built from this oversight is flawed. A straightforward fix to the analysis is achieved through the correct assignment of the two structures; resolving that issue introduces a disagreement between the theory and relevant experimental observations, throwing doubt on the methodology as a whole. Can the mismatch between the formally correct analysis and experimental observations be rectified? Or is the analysis logically sound, but not consistent with nature?

Introduction

How do the crystal structures compare?

What are the implications for the physical model?