Metal-free enantiomorphic perovskite [dabcoH2]2+[H3O]+Br- 3 and its one-dimensional polar polymorph.

The structure and stoichiometry of a new metal-free and ammonium-free compound [dabcoH2]2+H3O+Br- 3 (where [dabcoH2]2+ = 1,4-di-aza-bicyclo-[2.2.2]octane dication) correspond to the general formula ABX 3 characteristic of perovskites. In enantiomorphic trigonal polymorph α of [dabcoH2]2+H3O+Br- 3, the corner-sharing [H3O]Br6 octahedra combine into a 3D framework embedding [dabcoH2]2+ dications in pseudo-cubic cages. In the more dense polymorph β, the face-sharing [H3O]Br6 octahedra form 1D polyanionic columns separated by [dabcoH2]2+ dications. These different topologies correlate with different crystal fields around the cations and their different disorder types: orientational disorders of [dabcoH2]2+ dications and H3O+ cations in polymorph α and positional disorder of [H3O]+ cations in polymorph β. The orientational disorder increases the lengths of OH⋯Br hydrogen bonds in polymorph α, but NH⋯Br distances of ordered dabcoH2 dications are longer in polymorph β. The presence of polar [H3O]+ cations in [dabcoH2]2+H3O+Br- 3 polymorphs offers additional polarizability of the centres compared with analogous metal-free [dabcoH2]2+[NH4]+Br- 3 perovskite. © Budzianowski et al. 2022.

Introduction

Perovskites constitute a wide group of crystalline materials with the characteristic formula ABX 3 and structures built from corner-sharing BX 6 octahedra and A cations in the cubic voids. The importance of perovskites is connected with their properties such as ferroelectric and relaxor properties (Strukov & Levanyuk, 1998 ▸), and numerous applications, for example as digital memories (Scott, 2000 ▸), sensors or photovoltaics (Szafrański & Katrusiak, 2017 ▸, 2016 ▸). The mineral CaTiO3, discovered in 1839 by Gustav Rose in the Ural mountains was named perovskite, after mineralogist Lev Alekseyevich von Perovski. Later, the name perovskite was used to describe a wider group of minerals with analogous structures and the general formula ABO 3, then extended to ABX 3, where X was a halide anion. Finally, organic–inorganic hybrid perovskites were designed, with large complex unit cells, and which still correspond to the networks built of corner-sharing octahedra or the octahedra corners connected through organic linkers, sometimes of considerable size (Boström & Goodwin, 2021 ▸). Also, 2D and 1D perovskite analogues differentiated in the composition (e.g. ABO 4, CsPb2Br5), ionicity and topologies (e.g. corner/edge/face-sharing octahedra) are also often described as perovskite analogues. Properties and applications of perovskite materials are often connected with their phase transitions and symmetry changes at phase transitions, involving ionic displacements or tilts of the BX 6 octahedra (Glazer, 1972 ▸; Howard & Carpenter, 2010 ▸; Carpenter & Howard, 2009 ▸). They can induce spontaneous polarization and ferroelectricity of crystals, such as for example in BaTiO3 and PbTiO3 (Megaw, 1946 ▸, 1952 ▸; Shirane et al., 1950 ▸; Shirane & Takeda, 1952 ▸; Shirane & Pepinsky, 1953 ▸; Nelmes & Kuhs, 1985 ▸). In some structures, the symmetry and properties are connected with the disorder of the ions. In recent years, metal-free perovskites are sought for their applications in sensors, detectors, light-emitting diodes (LEDs), photovoltaics and, generally, optoelectronics (Song et al., 2020 ▸; 2021a ▸; 2021b ▸). The main advantages of organic and hybrid organic–inorganic substitutes of ceramic perovskites are their reduced toxicity, owing to the absence of heavy metals, lower cost of production and processing (formation of thin layers, also in the flexible form) and their easier environment-friendly disposal and recycling. Owing to weaker cohesion forces, bio-friendly metal-free perovskites can exhibit increased sensitivity to external stimuli (Cui et al., 2021 ▸).

Recently, metal-free perovskites involving piperazine, dabco and ammonium cations (NH4+) were discovered (Bremner et al., 2002 ▸), and later their analogues [dabcoH]2+[NH4]+Br− 3 and [MdabcoH]2+[NH4]+Br− 3 (Mdabco stands for N-methyl­ated dabco, i.e. 1,4-di­aza­bicyclo­[2.2.2]octane, C6H12N2) were thoroughly investigated (Ye et al., 2018 ▸; Morita et al., 2020 ▸). These are considered environment-friendly and cheap alternatives to mineral perovskites (Li & Ji, 2018 ▸; Gao et al., 2021 ▸). At present, we report another metal-free perovskite compound [dabcoH2]2+[H3O]+Br− 3 obtained in the form of two polymorphs, α and β. Polymorph α has the structure of the analogous metal-free 3D perovskite [dabcoH2]2+[NH4]+Br− 3, where the [H3O]Br6 octahedra share vertices (Ye et al., 2018 ▸). In the structure of polymorph β, [H3O]Br6 octahedra share faces in 1D columns (Bremner et al., 2002 ▸; Ye et al., 2018 ▸). Both polymorphs α and β of [dabcoH2]2+[H3O]+Br− 3 are disordered, but in a different manner. It is characteristic that disorder effects are essential for the properties of many types of crystals, including perovskites and dabco monosalts, where the disorder is connected to the ferroelectric and relaxor properties (Szafrański & Katrusiak, 2004 ▸). Our present study is primarily aimed at identifying the structural features of the [dabcoH2]2+H3O+Br− 3 polymorphs.

Experimental

Single crystals of [dabcoH2]2+[H3O]+Br− 3, where [dabcoH2]2+ of the formula [C6H14N2]2+ stands for diprotonated 1,4-di­aza­bicyclo­[2.2.2]octane, were found as a small fraction of crystallizations aimed at growing relaxor ferroelectric dabcoH+ bromide (dabcoHBr) from the aqueous solution of dabco and HBr in a 1:1 equimolar ratio (Budzianowski & Katrusiak, 2006 ▸; Szafrański & Katrusiak, 2004 ▸). The X-ray diffraction studies of selected single crystals revealed the presence of a tri-component salt [dabcoH2]2+ hydro­nium tribromide, [dabcoH2]2+[H3O]+Br− 3 (polymorph α). Later, this compound (polymorph β) was obtained close to 100% yield by cooling and slowly evaporating the aqueous solution of dabco with the hydro­bromic acid at a 1:3 molar ratio (the initial crystallization of the equimolar dabco:HBr aqueous solution revealed polymorph α only). The single-crystal X-ray diffraction data (Table 1 ▸) were measured with a KUMA KM4-CCD diffractometer with a graphite-monochromated fine-focus Mo Kα tube and an Oxford Diffraction XCalibur R diffractometer with a fine-focus X-ray source from a Cu Kα tube and Ruby CCD detector; using the latest version of CrysAlis and CrysAlis PRO software (Rigaku OD, 2003 ▸, 2019a ▸). The crystal structure of polymorph α-[dabcoH2]2+[H3O]+Br− 3 was partly solved by direct methods in ShelxS-97 (Sheldrick, 2008 ▸) and then JANA (Petříček et al., 2014 ▸) produced the model. Because of the disorder in the structure, we attempted structural refinements in the lower-symmetry space groups with ShelxL (Sheldrick, 2015 ▸; Barbour, 2020 ▸; Hübschle et al., 2011 ▸) and JANA (Petříček et al., 2014 ▸). Finally, we established that polymorph α-[dabcoH2]2+[H3O]+Br− 3 crystallizes in the enantiomorphic trigonal space group P3221 (no indication of racemic twinning was detected); the refinement of its structure revealed disorder of the dabcoH2 dications in two orientations with nearly equal site-occupation factors of 0.53:0.47(2) (Table 1 ▸). Similar procedures were applied for solving and refining polymorph β-[dabcoH2]2+[H3O]+Br− 3 in the trigonal space group P3c1, where positional disorder was found for two of three symmetry-independent H3O+ cations; they are disordered at different rates, each in two sites located on a threefold axis (Table 1 ▸). The structure of polymorph β approximates the structure with a 3× smaller unit cell and the symmetry of the space group P 62c (cf. Table S1, Model 4 of the supporting information). The drawings of crystal structures were prepared with the programs Mercury (Macrae et al., 2020 ▸), POV-Ray (Barbour, 2020 ▸; Cason, 2004 ▸) and Vesta (Momma & Izumi, 2011 ▸). Selected structures were presented as autostereograms to facilitate their 3D perception (Katrusiak, 2001 ▸).

Table 1

Crystal data and experimental details for [dabcoH2]2+[H3O]+Br− 3 polymorphs α and β determined at 297 K and 0.1 MPa

Note the different rates of disorder of independent hydro­nium cations in polymorph β.

	α-polymorph	β-polymorph
Chemical formula moiety	[C6H14N2]·3Br+·H−O+	[C6H14N2]·3Br+·H−O+
Disordered cation/ratio	dabcoH2+ 0.53:0.47(2) orientation, H3O+ disordered in 2 half-occupied orientations	2H3O+ cations positionally disordered, each in 2 sites, 0.51:0.49(3), 0.84:0.16(1)
Empirical formula	C6H17Br3N2O	C6H17Br3N2O
Formula weight	372.94	372.94
Crystal system	Trigonal	Trigonal
Space group (No.)	P3221 (154)	P3c1 (158)
a, b, c (Å)	9.5838(3), 9.5838(3), 23.2270(8)	16.0425(1), 16.0425(1), 7.9666(7)
α, β, γ (°)	90, 90, 120	90, 90, 120
Volume (Å3)	1847.56(13)	1775.61(16)
Z/Z′	6/1	6/(3 × 1/3)
V/Z	307.93(2)	295.94(3)
Dx (g cm−3)	2.011	2.093
Wavelength (Å)	0.71073	1.54184
θ range (°)	2.454–29.253	3.181–71.234
Min/max indices h, k, l	−13/12, −9/12, −31/30	−19/19, −19/19, −9/9
F(000)	1080	1080
Reflections (all)	14765	37462
Independent reflections/R int	3159/0.0526	2269/0.0573
θ to 100% completeness (°)	25.242	67.684
Max/min transmission	0.985/0.956	0.334/0.060
Data/restraints/parameters	3159/84/201	2269/13/131
GooF on F 2	1.077	1.053
Final R 1/wR 2 Σ[I > 2σI]	0.0415/0.652	0.0287/0.0782
R 1/wR 2 (all data)	0.0705/0.740	0.0319/0.0814
Extinction coefficient	–	0.00143(13)
Absorption coefficient (mm−1)	9.792	12.368
Absolute structure parameter	0.005(15)	−0.02(2)
Max diff. peak/hole [eÅ−3]	0.548/−0.607	0.443/−0.468

The final crystal and structural data and experimental details for both polymorphs are summarized in CIF format in the Cambridge Crystallographic Database Centre as supplementary publications 2132130 and 2132131. They can be obtained free of charge from the Cambridge Structural Database at https://www.ccdc.cam.ac.uk/structures/.

Discussion

The structure of polymorph α-[dabcoH2]2+[H3O]+Br− 3, where the corner-sharing [H3O]Br6 octahedra are connected in a 3D framework occluding dabcoH2 dications (Fig. 1 ▸) clearly corresponds to the classical perovskite structures of the formula ABBr3 (Glazer, 1972 ▸; Megaw, 1946 ▸). Moreover, the symmetry of the polymorph α-[dabcoH2]2+[H3O]+Br− 3 can be connected to the tilts of the [H3O]Br6 octahedra, consistent with Glazer’s code a - for mineral perovskites. However, due to the non-spherical symmetry of H3O+ and [dabcoH2]2+ cations, the unit-cell volume is increased and the crystal symmetry of α-[dabcoH2]2+[H3O]+Br− 3 is lowered to one of the enantiomeric space groups P3221 or P3121. The trigonal unit cell (Z = 6) of α-[dabcoH2]2+[H3O]+Br− 3 comprises six prototypic perovskite pseudo-rhombohedral sub-units (Z′ = 1). An average prototypic pseudo-rhombohedral unit (pR) can be represented in terms of the trigonal (Tr) unit vectors according to the matrix (cf. Figs. 1 ▸ and 2 ▸): where the vector indices refer to lattices Tr and pR. This transformation yields the idealized prototypic rhombohedral unit cell, of the dimensions a pR = 6.753 Å and αpR = 90.40°, close to the average of the true dimensions of the pseudo-rhombohedral cell: a pR = 6.720, b pR = 6.784, c pR = 6.755 Å, αpR = 90.12°, βpR = 90.67° and αpR = 90.42° (cf. Figs. 1 ▸ and 2 ▸). The reverse transformation, from the prototypic rhombohedral sub-unit pR to the trigonal unit cell Tr, is In ABX 3 perovskite structures the interactions between cations (B) and anions (X) forming the 3D polyanionic framework [BX 3] are mainly electrostatic, like those between the framework and cations A contained in the cages. The shortest contacts in the structure of α-[dabcoH2]2+[H3O]+Br− 3 confirm that the H3O+ cations are OH⋯Br− bonded into the 3D framework (Table 2 ▸). The shortest distances C—H⋯Br− are listed in Table 3 ▸. Also, the [dabcoH2]2+ cations are NH⋯Br− bonded to the linker anions, and these hydrogen bonds are somewhat shorter than those assigned to the framework. Such short contacts between the central cation and the atoms that form the cages are often present in perovskite structures and are considered a significant contribution to the stability and stiffness of the crystals (Ciupa-Litwa et al., 2020 ▸; Collings et al., 2016 ▸; Scatena et al., 2021 ▸; Adjogri & Meyer, 2020 ▸; Hou et al., 2020 ▸).

Figure 1

One pseudo-cubic subunit of the trigonal polymorph α-[dabcoH2]2+[H3O]+Br− 3, extracted from the 3D network of corner-sharing [H3O]Br6 octahedra (cf. Figs. S4, S5 and S6–S9). Colour and size code: large brown spheres Br, medium blue N, red O, medium brown C, small white H; two colours indicate the partial occupation of disordered N and C atoms.

Figure 2

Autostereographic view (Katrusiak, 2001 ▸) of the structure of α-[dabcoH2]2+H3O+Br− 3. Hydrogen bonds are indicated by cyan lines. Colour code: brown Br, red O; for disordered cations their different positions are distinguished by two colours: light and dark blue N, light and dark grey C; light-grey and black H.

Table 2

Hydrogen-bond contacts in α-[dabcoH2]2+[H3O]+Br− 3, with the donor–acceptor (D⋯A) distances shorter than the sum of their van der Waals radii (Bondi, 1964 ▸) and angle D—H⋯A larger than 110°

D—H⋯A	D—H (Å)	H⋯A (Å)	D⋯A (Å)	D—H⋯A (°)
O(2w)—H(21w)⋯Br(1A)i	0.8201(14)	2.564(16)	3.370(5)	168(6)
O(2w)—H(22w)⋯Br(2A)ii	0.8201(14)	2.62(2)	3.394(4)	158(6)
O(2w)—H(23w)⋯Br(3A)iii	0.8201(15)	2.625(16)	3.4303(12)	167(6)
O(1w)—H(11w)⋯Br(3A)iv	0.8201(14)	2.62(2)	3.411(5)	162(6)
O(1w)—H(12w)⋯Br(1A)	0.8201(14)	2.59(2)	3.3591(9)	156(5)
O(1w)—H(13w)⋯Br(2A)v	0.8201(15)	2.639(19)	3.439(5)	166(6)
N(1A)—H(1A)⋯Br(2A)	0.98	2.39	3.244(13)	145.0
N(2A)—H(2A)⋯Br(3A)	0.98	2.37	3.240(14)	148.1
N(1B)—H(1B)⋯Br(2A)	0.98	2.36	3.249(15)	150.1
N(2B)—H(2B)⋯Br(3A)	0.98	2.36	3.243(16)	149.0

Symmetry codes: (i) 1 − x, y − x, 2/3 − z; (ii) x −y + 1, −y + 1, 1/3 − z; (iii) 2 − x, 1 − x + y, 2/3 − z; (iv) y, x, 1 − z; (v) −x, y − x, 2/3 − z.

Table 3

Shortest interionic contacts corresponding to hydrogen bonds NH⋯Br and OH⋯Br in β-[dabcoH2]2+[H3O]+Br− 3; cf. Table 2 ▸

D—H⋯A	D—H (Å)	H⋯A (Å)	D⋯A (Å)	D—H⋯A (°)
O(1w)—H(1w)⋯Br(1)	0.82(2)	2.31(3)	3.121(4)	169(12)
O(2w)—H(2w)⋯Br(2)	0.83(2)	2.6(2)	3.102(8)	119(18)
O(1wA)—H(1wA)⋯Br(1)	0.83(2)	2.23(11)	3.037(15)	165(40)
O(2wA)—H(2wA)⋯Br(2)	0.83(2)	2.33(4)	3.148(6)	171(17)
O(3w)—H(3w)⋯Br(3)	0.84(3)	2.30(5)	3.079(4)	154(9)
N(1)—H(1)⋯Br(1)	0.98	2.82	3.541(6)	130.7
N(1)—H(1)⋯Br(2)	0.98	2.96	3.602(7)	124.2
N(1)—H(1)⋯Br(3)	0.98	3.22	3.837(5)	122.2
N(2)—H(2)⋯Br(1)i	0.98	3.18	3.787(6)	121.6
N(2)—H(2)⋯Br(2)i	0.98	3.06	3.734(7)	126.8
N(2)—H(2)⋯Br(3)i	0.98	2.79	3.497(5)	129.5
C(1)—H(1A)⋯Br(2)	0.97	2.97	3.598(6)	123.4
C(1)—H(1B)⋯Br(1)ii	0.97	2.95	3.738(7)	139.4
C(2)—H(2A)⋯Br(1)	0.97	3.00	3.521(7)	115.4
C(2)—H(2B)⋯Br(3)iii	0.97	3.08	3.817(7)	133.9
C(3)—H(3A)⋯Br(3)	0.97	2.96	3.635(6)	127.5
C(3)—H(3B)⋯Br(2)iv	0.97	3.12	3.840(7)	132.0
C(4)—H(4A)⋯Br(1)ii	0.97	3.12	3.838(7)	131.8
C(4)—H(4B)⋯Br(2)i	0.97	3.00	3.593(6)	120.9
C(5)—H(5A)⋯Br(3)iii	0.97	3.01	3.781(7)	137.1
C(5)—H(5B)⋯Br(1)i	0.97	2.98	3.665(6)	129.0
C(6)—H(6A)⋯Br(2)iv	0.97	2.97	3.751(6)	138.8
C(6)—H(6B)⋯Br(3)i	0.97	3.00	3.553(6)	117.2

Symmetry codes: (i) y, x, z − 1; (ii) x, x − y + 1, z − 1/2; (iii) 1 − y, 1 − x, z − 1/2; (iv) 1 − x + y, y, z − 1/2.

All hydrogen donors in the structure of α-[dabcoH2]2+[H3O]+Br− 3 are disordered. The hydro­nium H3O+ cations are orientationally disordered in two positions around the oxygen atom. The three hydrogen atoms are located in six half-occupied sites, consistent with the H3O+ dimensions (Lundren & Olovsson, 1976 ▸), as shown in Figs. 1 ▸, 2 ▸ and S1–S8 (Table S2). Each of the six half-occupied hydrogen sites is involved in one OH⋯Br− hydrogen bond [Figs. S3(a) and S3(b)]. Dication [dabcoH2]2+ is disordered in two orientations. The dication is involved in two OH⋯Br− hydrogen bonds. Each of these hydrogen bonds is split between two half-occupied hydrogen sites (Fig. 2 ▸). There are also short distances (∼2.8 to ∼3 Å) between Br ions and methyl­ene hydrogen atoms. All hydrogen bonds listed in Table 2 ▸ can be classified as weak interactions, which are consistent with the considerable disorder of this structure. Like for the mineral perovskites, the symmetry of the crystal field around the cations plays a crucial role in their disorder.

In addition to the trigonal polymorph α-[dabcoH2]2+[H3O]+Br− 3, we found another concomitant trigonal polymorph β-[dabcoH2]2+[H3O]+Br− 3 (Table 1 ▸, Fig. 3 ▸). In its structure, the octahedra of hydro­nium cations and bromine anions ([H3O]Br6) are face-to-face arranged into columns and the [dabcoH2]2+ dications are located between these columns. However, in contrast to polymorph α, in polymorph β-[dabcoH2]2+[H3O]+Br− 3 the [dabcoH2]2+ dications are ordered in general positions, while out of three independent hydro­nium cations [H3O]+ two are disordered. Each of these two [H3O]+ cations is disordered between a pair of sites along the same column along z, as illustrated in Fig. 4 ▸. The distance between the pairs of partially occupied oxygen sites (Table 1 ▸) is approximately equal to 1/4 of the unit-cell parameter c, i.e. about 2 Å. It is intriguing that the oxygen sites disordered along the threefold axes lie off the centre of the [H3O]+Br6 octahedra, while the centres of the octahedra are at the midpoints between the oxygen sites; the disordered [H3O]+ cation is also located off its octahedron centre (Fig. 4 ▸). These off-centre sites of the oxygen atoms result in O⋯Br distances (Table 3 ▸) significantly shorter compared with those in α-[dabcoH2]2+[H3O]+Br− 3 (Table 2 ▸). The off-centre positional disorder of hydro­nium cations in β-[dabcoH2]2+[H3O]+Br− 3 suggests that the OH⋯Br hydrogen bonds favour shorter distances than those between the bromine anion and the octahedron centre. The longer O⋯Br distances in α-[dabcoH2]2+[H3O]+Br− 3 can be attributed to the orientational dynamic disorder of the H3O+ cation; its hydrogen atoms share the tetrahedrally located sites with the lone-electron pair: the tetrahedral sites do not match the octahedral locations of the bromine anions around, while the lone-electron pair does not contribute to the attraction of the hydrogen bonds, but it can be associated with some repulsion. On the other hand, the [dabcoH2]2+ dications in β-[dabcoH2]2+[H3O]+Br− 3 are stabilized in the ordered position by their trigonal environment and form NH⋯Br− contacts that are significantly longer than those of disordered [dabcoH2]2+ dications in the pseudo-cubic crystal environment of α-[dabcoH2]2+[H3O]+Br− 3 (Tables 2 ▸ and 3 ▸).

Figure 3

Polymorph β-[dabcoH2]2+[H3O]+Br− 3 with chains of face-sharing octahedra: (a) parallel view along the [001] direction; and (b) autostereographic view (Katrusiak, 2001 ▸) comparing the columns of face-sharing octahedra – the full translational symmetry along the unit-cell diagonal across the page is indicated by two grey circles below the drawing. The colour code for atoms is the same as in Fig. 1 ▸.

Figure 4

Crystal structure of polymorph β-[dabcoH2]2+[H3O]+Br− 3 projected along (a) the [001] direction and (b) the [010] direction. Partially occupied sites of the disordered hydro­nium cations are indicated. Colour code: brown Br; blue N, red O; dark grey C; light grey H.

The crystallographic information about polymorphs explains the origin of different disorder types in their structure. The [dabcoH2]2+ dications can assume either the D 3 twisted left or right propeller conformation, or the averaged D 3h symmetry (Olejniczak et al., 2013 ▸). However, in polymorph α the dications are located in the pseudo-cubic cages, approximating the O h symmetry. Consequently, the crystal environment of polymorph α does not stabilize one specific orientation of the cation. On the other hand, the trigonal symmetry of polymorph β results in the trigonal surrounding of the dications, matching their symmetry well. Indeed, the shape of pseudo-D 3h-symmetric dications is consistent with the trigonal crystal environment. In polymorph α, the hydro­nium cations are located at the vertices of the pseudo-cubic cages, which does not favour any of their displacements. In polymorph β, the [H3O]+ cations are located in the channel-like surrounding of the threefold axes, which does not appear to strongly favour any site along z and results in the disorder of the cations. Interestingly, the structure of polymorph β approximates the higher symmetry of space group P 62c, with the unit cell 3× smaller (Figs. S9–S16, Tables S1 and S2). This high symmetry is broken by small tilts of the dications and displacements of the Br anions, as well as by the differences between the hydro­nium cations, ordered and disordered to different extents (Table 1 ▸), as shown in the structure projections in Fig. 4 ▸.

Conclusions

Polymorphs of [dabcoH2]2+[H3O]+Br− 3 illustrate the universality of perovskite structures and their characteristic features. The general formula ABX 3 of perovskites, initially associated with minerals and ionic crystals, can clearly be extended to various hybrid organic–inorganic compounds, where organic central A cations interact with (in)organic X linkers, binding the B metal centres in the systems with much weaker cohesion forces, such as hydrogen bonds and electrostatic interactions between molecular ions. Even the metal-free compounds still display the characteristic structural properties of perovskites, controlled by the tilts of BX 6 octahedra and disorder of the cations: the size and orientation of the molecular ions are additional factors responsible for the crystal symmetry and macroscopic properties of the hybrid and metal-free perovskites. The structures of both polymorphs of [dabcoH2]2+[H3O]+Br− 3 are disordered under normal conditions, which is an indication of possible temperature and pressure-induced phase transitions of properties. Both orientational and positional disorder are present and they interplay with the crystal field around the cations and their hydrogen bond capabilities. Polymorph α-[dabcoH2]2+[H3O]+Br− 3, like its close analogue [dabcoH2]2+[NH4]+Br− 3, is one of very few enantiomorphic and polar perovskites reported so far. With the exception of C4H14N2RbCl3 (Paton & Harrison, 2010 ▸), the previously reported enantiomorphic perovskites employed chiral cations, for example in (R)-, (S)-3-(fluoro­pyrrolidinium)MnBr3 and in (R)-, (S)-N,N-di­methyl-3-fluoro­pyrrolidinium CdCl3 (Gao et al., 2020 ▸; Peng et al., 2021 ▸), where the enantiomorphic form of the crystal was permanently connected with the chiral cation used for the synthesis. To our knowledge, the polymorphs [dabcoH2]2+[NH4]+Br− 3 and α-[dabcoH2]2+[H3O]+Br− 3 are the first metal-free enantiomorphic and polar 3D perovskite structures where no permanent chiral cations are present and therefore this structure can be switched between two enantiomorphs. The structure of polymorph β-[dabcoH2]2+[H3O]+Br− 3 is polar, which can also potentially result in ferroelectric properties. Thus, in both polymorphs, the substitution of the non-polar [NH4]+ cation with the polar [H3O]+ cation can result in increased polarizability of the system and advantageous switchable properties desired for practical applications in optoelectronic devices.

Related literature

The following references are cited in the supporting information: Rigaku OD (2019b ▸, 2020 ▸, 2021 ▸); Clark & Reid (1995 ▸); Laetsch & Downs (2006 ▸).

Crystal structure: contains datablock(s) dhbr1af_41001, dabco1_h168b6_abs2. DOI: 10.1107/S2052252522006406/ct5016sup1.cif

Supporting figures and tables. DOI: 10.1107/S2052252522006406/ct5016sup2.pdf

CCDC references: 2132130, 2132131