BaZrSe3: Distorted Chalcogenide Perovskite

• BaZrSe3 is a Ba-based ternary chalcogenide defined by a distorted perovskite framework (Pnma) with a 1.0 eV direct bandgap.
• It exhibits strong visible-light absorption and promising p-type dopability via Ba-site substitutions like Cs and Cu for photovoltaic absorbers.
• Computational studies highlight defect compensation challenges and dopant mobility issues, underpinning its potential for tandem solar cell applications.

ZrBaSe$_3$, more commonly written in the cited literature as BaZrSe$_3$, is a Ba-based ternary chalcogenide with the same stoichiometry and chemistry under either element-ordering convention. In the Materials Project database it is identified as mp-998427, and it is treated as an orthorhombic distorted perovskite-type chalcogenide in space group $Pnma$. Within the available literature, its most explicit characterization comes from first-principles photovoltaic screening, which identifies it as a 3D bulk solid with a direct HSE06 bandgap of $1.00~\text{eV}$, strong visible-spectrum absorption, and promising p-type dopability for single-junction and tandem-cell absorber applications (Raghupathy et al., 26 Aug 2025). Much of the deeper microscopic interpretation of lattice polarizability, phonon coupling, and epitaxial stabilization is presently contextual, inferred from closely related Ba–Zr–S chalcogenide perovskites rather than reported directly for BaZrSe$_3$ itself (Filippone et al., 2020).

1. Identity, nomenclature, and crystal framework

The compound is referred to consistently as BaZrSe$_3$ in the photovoltaic screening study, while ZrBaSe$_3$ denotes the same stoichiometry. It is described there as a distorted perovskite of formal $ABX_3$ type, with Ba on the $A$ site, Zr on the $B$ site, and Se on the $_3$0 site. The reported crystal symmetry is $_3$1, and the material is treated as a 3D bulk solid rather than as a layered or molecular phase (Raghupathy et al., 26 Aug 2025).

Its structural motif is the standard chalcogenide-perovskite one: Zr is octahedrally coordinated by Se in $_3$2 octahedra, these octahedra are corner-sharing within a network, and Ba occupies the larger cavities of that framework. The study does not provide explicit lattice constants or atomic coordinates for BaZrSe$_3$3, but it places the compound within the broader $_3$4 chalcogenide perovskite family examined in prior work (Raghupathy et al., 26 Aug 2025).

A recurrent point of clarification is that the most direct structural evidence in the supplied literature pertains to the sulfide analogue BaZrS$_3$5, which is also $_3$6 and a distorted perovskite with corner-sharing $_3$7 octahedra (Sadeghi et al., 2021). This does not by itself constitute a measurement on BaZrSe$_3$8, but it provides the principal comparative framework for interpreting ZrBaSe$_3$9.

2. Electronic structure and optical response

The clearest reported electronic metric for BaZrSe$Pnma$0 is its HSE06 bandgap,

$Pnma$1

which places it exactly at the lower edge of the $Pnma$2–$Pnma$3 screening window used for photovoltaic absorbers. The same study states that the promising compounds identified there are predicted to possess direct bandgaps at the $Pnma$4 point; BaZrSe$Pnma$5 is one of those compounds, so both the VBM and CBM are at $Pnma$6 (Raghupathy et al., 26 Aug 2025).

The valence-band edge is described qualitatively as deriving mainly from relatively localized Se $Pnma$7 orbitals, which produces a less dispersive valence band. No explicit effective masses are reported for BaZrSe$Pnma$8, and the conduction-band composition is not explicitly given in the text presented. A plausible implication, following the comparative discussion in the source, is that the conduction manifold has substantial Zr $Pnma$9 and Se $1.00~\text{eV}$0 character, but that point is contextual rather than directly tabulated for this compound (Raghupathy et al., 26 Aug 2025).

Optically, the material is reported to exhibit strong absorption in the visible spectrum, comparable to CuInSe$1.00~\text{eV}$1 and MAPbI$1.00~\text{eV}$2. The calculated spectrum shows a sharp onset at $1.00~\text{eV}$3 and large $1.00~\text{eV}$4 across visible energies, although numerical absorption coefficients are not tabulated in the text provided. Given the direct gap at $1.00~\text{eV}$5 and the Se $1.00~\text{eV}$6-dominated VBM, the band-edge transitions are described as direct $1.00~\text{eV}$7 transitions with strong dipole-allowed character. This combination of a $1.00~\text{eV}$8 direct gap and strong visible/NIR absorption is the basis for its classification as a bottom-cell or near-optimal low-gap absorber (Raghupathy et al., 26 Aug 2025).

3. Defect thermodynamics, compensation, and p-type dopability

Defect physics is a central part of the computational assessment of BaZrSe$1.00~\text{eV}$9. The standard defect formation enthalpy used in that study is

$_3$0

with the usual meanings for $_3$1, $_3$2, $_3$3, $_3$4, $_3$5, $_3$6, and $_3$7. The calculations were performed with large supercells and finite-size/image-charge corrections (Raghupathy et al., 26 Aug 2025).

Under anion-rich, specifically Se-rich, growth conditions, the intrinsic defect landscape is reported as follows. $_3$8 is a donor-type defect with low formation energy when $_3$9 is near the VBM. $_3$0 is a neutral defect with low formation energy at p-type Fermi levels. $_3$1 has higher formation energy than $_3$2 at p-type Fermi levels, and $_3$3 has comparatively large formation energy and a large diffusion barrier. Other antisites and interstitials are reported to have higher formation energies and to be less likely to dominate (Raghupathy et al., 26 Aug 2025).

At low $_3$4, the lowest-formation-energy intrinsic defects are therefore $_3$5 and $_3$6. The principal compensation mechanism identified is $_3$7, which acts as a hole-killer donor when the Fermi level is pushed too close to the VBM. This means that p-type doping is not unconstrained: acceptor introduction must compete against selenium-vacancy compensation (Raghupathy et al., 26 Aug 2025).

The same work nevertheless concludes that BaZrSe$_3$8 has good p-type dopability. The key extrinsic dopants studied are Ba-site substitutions. $_3$9 has lower formation energy than $_3$0 and $_3$1, is energetically favorable under Se-rich, p-type conditions, and acts effectively as an acceptor through the formal substitution $_3$2. $_3$3 is more stable than $_3$4, is likely to form, and is likewise expected to be acceptor-like. The paper’s explicit conclusion is that BaZrSe$_3$5 has a suitable bandgap and good p-type dopability (Raghupathy et al., 26 Aug 2025).

Diffusion calculations add a device-relevant nuance. In BaZrSe$_3$6, $_3$7 has a lower migration barrier than $_3$8 and $_3$9, while $ABX_3$0 has a migration barrier smaller than those of the intrinsic vacancies. This suggests that dopant mobility, grain-boundary segregation, and defect redistribution may be important under processing or operating conditions (Raghupathy et al., 26 Aug 2025).

4. Stability, screening methodology, and computational treatment

BaZrSe$ABX_3$1 emerged from a broader computational survey of Ba-based ternary chalcogenides for photovoltaic applications. The total number of Ba-based ternary chalcogenides in the Materials Project database was reported as 279, and screening based on bandgap size and stability reduced this to 19 compounds. Among these, two compounds—$ABX_3$2 and $ABX_3$3—were identified as promising absorbers for single-junction and tandem cells and were investigated in detail (Raghupathy et al., 26 Aug 2025).

The stability criterion reported for selected candidates is $ABX_3$4, and BaZrSe$ABX_3$5 is described as earth-abundant, stable, and less toxic than existing absorbers. The source further notes that previous studies by Ong et al. and Sun et al. predicted BaZrSe$ABX_3$6 to be stable at room temperature. A cautious reading is that the computational evidence places it on or close to the convex hull within the Ba–Zr–Se chemical space, but the detailed phase diagram is not reproduced in the supplied text (Raghupathy et al., 26 Aug 2025).

The computational workflow is specified in considerable detail. The calculations use VASP with PAW pseudopotentials, a plane-wave cutoff of $ABX_3$7, and geometry optimization converged when forces are $ABX_3$8. Screening used PBE, whereas final electronic structure and optical properties used the HSE06 screened hybrid functional. Brillouin-zone sampling employed a $ABX_3$9-centered Monkhorst–Pack mesh controlled through the VASP KSPACING tag. Defect calculations were automated with PyCDT, refined at HSE06 for promising compounds including BaZrSe$A$0, and corrected using Freysoldt/Kumagai finite-size and image-charge schemes. Optical properties were obtained from HSE06 within the independent-particle approximation using the Gajdoš et al. formalism. Vacancy-mediated diffusion was treated by CI-NEB with 5 images per path and a spring constant of $A$1 (Raghupathy et al., 26 Aug 2025).

5. Relation to BaZrS$A$2: dielectric polarizability, phonons, and polarons

The most detailed microscopic picture available for the Ba–Zr–chalcogen perovskite family comes from the sulfide-side study of BaZrS$A$3 and Ba$A$4Zr$A$5S$A$6. That work establishes them as semiconductors with low-frequency relative dielectric constant in the range $A$7–$A$8 and band gap in the range $A$9–$B$0. For single crystals, impedance spectroscopy gives $B$1 for BaZrS$B$2 and $B$3 for Ba$B$4Zr$B$5S$B$6, with values nearly frequency- and temperature-independent in the measured range. DFPT attributes the larger polarizability of the perovskite primarily to enhanced IR mode-effective charges and softer phonon behavior, especially along $B$7, rather than to large differences in Born effective charges alone (Filippone et al., 2020).

The same study estimates a sizable Fröhlich coupling constant, with $B$8 for BaZrS$B$9 and $_3$00 for Ba$_3$01Zr$_3$02S$_3$03, and concludes that charge carriers are large polarons (Filippone et al., 2020). This is directly relevant because the supplied contextual material identifies ZrBaSe$_3$04 as the Se analogue of BaZrS$_3$05, with the same formal stoichiometry and very likely the same distorted perovskite structure. A plausible implication is that the same coupling between covalent Zr–chalcogen bonding, soft IR-active modes, and large ionic dielectric response may operate in BaZrSe$_3$06, although no direct dielectric tensor, impedance spectrum, or Fröhlich-$_3$07 value for BaZrSe$_3$08 is reported in the supplied sources (Filippone et al., 2020).

Additional context comes from single-crystal BaZrS$_3$09 spectroscopy. Room-temperature measurements on high-quality crystals show strong, band-to-band-dominated photoluminescence, a direct band gap of about $_3$10, and a monoexponential carrier lifetime of $_3$11. The work attributes the short lifetime primarily to strong electron–phonon coupling and phonon-assisted carrier decay, while also suggesting that partial cation or anion substitution could mitigate electron–phonon coupling and enhance carrier lifetimes (Nielsen et al., 20 Mar 2025). This suggests that BaZrSe$_3$12, as the anion-substituted analogue, should be examined not only for its favorable bandgap but also for its phonon-limited recombination dynamics.

6. Device role, synthesis context, and open questions

In the photovoltaic screening study, BaZrSe$_3$13 is assigned a specific device role. With $_3$14, strong absorption, and promising p-type dopability, it is proposed for single-junction solar cells and as a bottom-cell absorber in tandem cells. The suggested tandem pairing is $_3$15-BaCu$_3$16Se$_3$17 as the top-cell absorber with a bandgap of about $_3$18, and BaZrSe$_3$19 as the low-gap bottom absorber optimized for the near-infrared (Raghupathy et al., 26 Aug 2025).

The practical synthesis status is less explicit. The screening paper is computational, but it relies on the existence of BaZrSe$_3$20 in the Materials Project database and refers to prior ab initio and experimental studies. It also notes that chalcogenide perovskite synthesis has been demonstrated experimentally, including BaZr(S,Se)$_3$21-related compositions. This suggests that BaZrSe$_3$22 is not merely a hypothetical stoichiometry, but the supplied material does not provide a dedicated experimental synthesis protocol for bulk or thin-film BaZrSe$_3$23 (Raghupathy et al., 26 Aug 2025).

A relevant growth template is provided by molecular-beam-epitaxy work on BaZrS$_3$24. That study demonstrates single-step gas-source chalcogenide MBE of orthorhombic $_3$25 BaZrS$_3$26 thin films with near-perfect stoichiometry, atomically smooth surfaces, and atomically sharp interfaces. It further argues that epitaxy may stabilize high-selenium-content perovskites and outlines an alloy route through $_3$27 under epitaxial control (Sadeghi et al., 2021). For BaZrSe$_3$28, this does not amount to an achieved synthesis in the cited work, but it identifies a plausible pathway: epitaxial stabilization or compositionally graded access from the sulfide side.

Several open questions remain explicit. For BaZrSe$_3$29, no direct dielectric measurements, no explicit carrier effective masses, no experimental band structure, and no measured carrier lifetime are provided in the supplied corpus. The Se $_3$30-dominated, less dispersive valence-band edge indicates a possible mobility limitation on the hole side, while low-energy $_3$31 donors imply compensation pressure against p-type doping. Conversely, the direct $_3$32 gap, strong absorption, and favorable Ba-site acceptor chemistry make it one of the clearest Ba-based candidates for Pb-, Cd-, In-, and Ga-free thin-film absorber development (Raghupathy et al., 26 Aug 2025).