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Double perovskites A₂BB′X₆

meidnet/families/double_perovskite_a2bbx6.yaml is a second family written with the same ingredients as the perovskite one. It shows what a family file can express:

  • a 10-site prototype in the rhombohedral primitive cell of an fcc lattice (lattice: fcc_primitive: edge a/√2, angles 60°), A at (¼,¼,¼) and (¾,¾,¾), B at the origin, B′ at (½,½,½), six X;
  • four site groups with their own element lists and oxidation states (B: +1/+2, B′: +3/+4);
  • a cell-size rule a = 2 (r_B + r_B′ + 2 r_X) (one B–X–B′ chain per half cell edge);
  • rules that average radii over several groups: tolerance_factor: {A: A, B: [B1, B2], X: X}.
meidnet families double_perovskite_a2bbx6 --variant halide

Rock-salt ordered double perovskite A2BB'X6 (Fm-3m)  [variant: halide]
  formula pattern: A2B1B2X6   (10 atoms per cell)
  A: 9 candidate elements - Large cation in the cuboctahedral cavity
  B1: 15 candidate elements - First octahedral cation (usually +1 or +2)
  B2: 20 candidate elements - Second octahedral cation (usually +3 or +4)
  X: 4 candidate elements - Anion shared between neighbouring B and B' octahedra

The halide variant has 10,800 compositions; 305 pass every rule (Cs₂AgBiI₆ among them). Candidates are built in the 10-atom primitive cell; the symmetry refinement step writes them out as the conventional 40-atom Fm-3m cell, which is what the CIF files contain.

Using it

A model for this family must be trained on double-perovskite structures aligned to the prototype:

family: double_perovskite_a2bbx6
data:
  table: double_perovskites.csv
  properties: [{column: band_gap, unit: eV}]
  align_to_prototype: true
generation:
  family: double_perovskite_a2bbx6
  variant: halide
  objectives: [{property: band_gap}]
  targets: [{band_gap: 2.0}]

The published checkpoint

The Perov-5 model was trained on 5-atom ABX₃ cells. The Studio will score double-perovskite compositions with it (the encoder accepts any cell up to 20 atoms), but those predictions are far outside its training distribution — a demonstration of the mechanics, not a result.