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Magneto-structural phase transitions in Ba1-xAxFe2As2 (A = K, Na) materialsare discussed for both magnetically and orbitally driven mechanisms, using asymmetry analysis formulated within the Landau theory of phase transitions.Both mechanisms predict identical orthorhombic space-group symmetries for thenematic and magnetic phases observed over much of the phase diagram, but theypredict different tetragonal space-group symmetries for the newly discoveredre-entrant tetragonal phase in Ba1-xNaxFe2As2 (x ~ 0.24-0.28). In a magneticscenario, magnetic order with moments along the c-axis, as foundexperimentally, does not allow any type of orbital order, but in an orbitalscenario, we have determined two possible orbital patterns, specified byP4/mnc1' and I4221' space groups, which do not require atomic displacementsrelative to the parent I4/mmm1' symmetry and, in consequence, areindistinguishable in conventional diffraction experiments. We demonstrate thatthe three possible space groups are however, distinct in resonant X-ray Braggdiffraction patterns created by Templeton & Templeton scattering. This providesan experimental method of distinguishing between magnetic and orbital models.

Symmetry of re-entrant tetragonal phase in Ba1-xNaxFe2As2: Magnetic versus orbital ordering mechanism