Polyphase IIR halfband decimator using biquad stages

Using DSP/Signal Toolbox.

I am trying to use matlab to generate coefficients for polyphase IIR halfband decimator. Which can be achieved using :

    %%Halfband filter (-3dB at 0.5), Transition band 0.1 wide and 80dB stopband attenuation
    f  = fdesign.decimator(2, 'halfband','TW,Ast' ,0.1,80);
    h1 = design(f,'ellip');
    h1.realizemdl

This generates:

However refactoring phase 1 to see the structure it looks like a cascade of first order df1 stages. Phase 2 is just a 0.5 gain.

The filter structure I am targeting is implemented as cascaded biquads. Phase 1 of a higher order than Phase 2.

*How do I get matlab to generate polyphase IIR filters using biquad stages?* where Phase 2 is more than a gain step. Ideally using the ellip method.

I have tried a few things like:

    He3 = design(f,'ellip','FilterStructure','cascadewdfallpass');
    He3.realizemdl

Which creates a Phase 2 with a real filter but not using biquad stages.

I am not aware of how to find which 'FilterStructures' are valid but found a reference to 'df2sos', which does not seem compatible with the ellip moethods.

    He3 = design(f,'ellip','FilterStructure','df2sos');
    Error using fdesign.decimator/ellip (line 19)
    'df2sos' is an invalid structure for this design.

I have been using Digital Filters with MATLAB by Ricardo A. Losada as my reference for this.

I have also asked this question on DSP.stackexchange.