Table 2.

Literature survey on the polarity of OIHPs.

Technique	Nonpolar I4/mcm	Polar I4cm	Noncommittal
X-ray and neutron diffractions	Refs [5,42–45]	Refs [6,41]	Ref. [40]
Optic SHG	Refs [12,19,62,67]	Refs [76,93]
Macroscopic measurements	Refs [59,60,64]	Refs [48,50,76,90,101]	Ref. [75]
Microscopic PFM	Refs [8,63,65,68,91]	Refs [46,47,49–58,83]	Refs [61,80,81,104]
TEM		Ref. [73]	Ref. [70]
DFT and MD simulations	Refs [12,19,66]	Ref. [92]	Refs [8,21]