Research ArticleMATERIALS SCIENCE

Negative-pressure polymorphs made by heterostructural alloying

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Science Advances  20 Apr 2018:
Vol. 4, no. 4, eaaq1442
DOI: 10.1126/sciadv.aaq1442
  • Fig. 1 Stabilization of negative-pressure polymorphs in heterostructural alloys.

    (A) Schematic illustration of α-, β-, and γ-polymorph enthalpies ΔH as a function of their respective atomic volume V and polymorph energies ΔEA for a model compound A. Stabilization of lower-density polymorph γ would require negative pressure (p = −∂H/∂V < 0). (B) Polymorph enthalpies as a function of the alloying composition x in an alloy system A1−xBx at equilibrium volume. Because of a smaller nonideal component of the mixing enthalpy ΔHΩ,γ < ΔHΩ,α, the lower-density γ polymorph can become energetically favorable for intermediate alloying concentrations, if entropic stabilization or kinetic barriers prevent phase separation into α and β. (C) Grayscale projection of the minimum enthalpy ΔHmin(V,x) among the considered polymorphs outlines the basins of attraction for the three considered structures. The smaller bowing of the γ-polymorph enthalpy enables the stabilization of this high-volume structure by controlling the alloying concentration. a.u., arbitrary units.

  • Fig. 2 Stabilization of the WZ polymorph of the Mn(Se,Te) alloy.

    (A) Calculated minimum enthalpy ΔHmin of MnSe1−xTex alloys on the color scale as a function of volume V and composition x. For intermediate compositions, the higher-volume WZ structure becomes energetically favorable compared to lower-volume RS and NC structures, like under negative-pressure conditions. (B) Measured false-color plot of XRD intensities as a function of composition for the MnSe1−xTex films deposited at 320°C. For intermediate compositions, the Mn(Se,Te) films crystallize mostly in the WZ structure, whereas the MnSe and MnTe parent compounds crystallize in RS and NC structures.

  • Fig. 3 Experimental crystal structure analysis of MnSe1−xTex alloys.

    (A) Synchrotron XRD measurements of MnSe0.5Te0.5 thin films grown on glass at 320°C substrate temperature [black circle with white boarder in (B)] confirm the stabilization of the high-enthalpy, low-density WZ polymorph. Trace amounts of MnTe NC (*) and MnSe RS (#) could be present in the film. The top and the bottom panels show the simulated XRD patterns of MnSe and MnTe in WZ and other structures, and the dashed lines are extrapolations of the WZ peaks. (B) Color-scale map of the WZ phase fraction for the sputter-deposited MnSe1−xTex thin films on glass. For intermediate compositions and lower deposition temperatures, the MnSe1−xTex films crystallize predominantly in the WZ structure with some RS- and NC-type impurities. Shaded areas represent single-phase regions of RS-MnSe and NC-MnTe determined by the disappearing phase method.

  • Fig. 4 Theoretical electronic and crystal structures of the Mn(Se,Te) materials.

    (A) MnSe in the RS crystal structure, (B) MnSe0.5Te0.5 in the WZ crystal structure, and (C) MnTe in the NC crystal structure. The larger band gap and smaller electron effective mass of the WZ-Mn(Se,Te) alloy result from weaker p-d hybridization of the Mn d-states of eg symmetry with the Se,Te p-states. In turn, the weaker hybridization is caused by tetrahedral coordination of the lower–atomic density WZ structure compared to the octahedral coordination of the RS-MnSe and NC-MnTe structures.

  • Table 1 Properties for the WZ-type MnSe0.5Te0.5 alloys compared to the RS-type MnSe and NC-type MnTe parent compounds.

    Listed are the calculated direct (d) or indirect (i) electronic band gaps (Egel), the hole and electron effective masses (me* and mh*), the calculated and measured piezoelectric coefficient (d33), and experimentally determined values for the optical band gaps (Egopt). The results of experimental measurements of piezoelectric response and electrical resistivity are provided in figs. S2 and S3.

    Materialsd33
    (pm/V)
    d33
    (pm/V)
    me*/m0mh*/m0Egel
    (eV)
    Egopt
    (eV)
    PolymorphTheoryExperimentTheoryTheoryTheoryExperiment
    RS-MnSe0.781.572.52 (i)2.50
    WZ-MnSe0.5Te0.59.541.5–110.301.802.71 (d)2.70
    NC-MnTe1.261.540.98 (i)1.45

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/4/eaaq1442/DC1

    Supplementary Methods

    Supplementary Additional Results

    table S1. Polymorph energies from DFT and RPA calculations and the magnitude of zero-point energies from the QHA.

    fig. S1. Detailed XRD study performed on MnSe0.5Te0.5 grown on indium tin oxide (ITO)–coated Eagle XG glass.

    fig. S2. PFM measurements of a WZ-MnSe0.5Te0.5 film grown on conductive ITO substrate.

    fig. S3. Optoelectronical characterization of Mn(Se,Te) alloys.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Methods
    • Supplementary Additional Results
    • table S1. Polymorph energies from DFT and RPA calculations and the magnitude of zero-point energies from the QHA.
    • fig. S1. Detailed XRD study performed on MnSe0.5Te0.5 grown on indium tin oxide (ITO)–coated Eagle XG glass.
    • fig. S2. PFM measurements of a WZ-MnSe0.5Te0.5 film grown on conductive ITO substrate.
    • fig. S3. Optoelectronical characterization of Mn(Se,Te) alloys.

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