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What is the structure of $\ce{PH4Br}$? I would like to know which atom is bonded to which. Are all five atoms directly bonded to phosphorus?

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No, not in the solid state. The most recent crystal structure analysis suggests there are nearly ideal tetrahedra of phosphonium cations $\ce{PH4+}$ with bromine anions $\ce{Br-}$ in between bound via H-bonds [1, ICSD#23691]:

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Reference

  1. Schroeder, L. W.; Rush, J. J. The Journal of Chemical Physics 1971, 54 (5), 1968–1973. DOI 10.1063/1.1675127.
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  • $\begingroup$ andelisk, does PH4Br exist as a liquid or gas? $\endgroup$ Jan 15, 2018 at 20:36
  • $\begingroup$ @HolgerFiedler $\ce{PH4Br}$ sublimes at approx. 30 °C. $\endgroup$
    – andselisk
    Jan 15, 2018 at 20:51
  • $\begingroup$ andelisk So what should prevent me to imagine that phospine and hydrogen bromide still exist in the solide phase and they simply sorted in a crystalline order? (I’m really a newbie in chemistry, so sorry for the naive question.) $\endgroup$ Jan 15, 2018 at 21:12
  • $\begingroup$ @HolgerFiedler Well, you can imagine anything you want, but neutron diffraction suggests otherwise (BTW have you checked the paper cited?). $\endgroup$
    – andselisk
    Jan 15, 2018 at 21:20
  • $\begingroup$ Behind a paywall? Doesn’t the sublimation of you HBr2 point to a very week bonding. Or more to a very homogeneous mixture? $\endgroup$ Jan 16, 2018 at 5:21

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