Ferroelectric phase transition in hydrogen-bonded 2-aminopyridine phosphate (NC₄H₄NH₂) H₃PO₄

Abstract

A new crystal of 2-aminopyridine phosphate (NC4H4NH2)·H3PO4 has been grown and its x-ray structure and physical properties were studied. At room temperature the crystals are monoclinic, space group C2/c. The flat 2-aminopyridine cations are hydrogen bonded to the anionic [PO4 ] groups. The interesting feature of the crystal structure is the three-dimensional network of hydrogen bonds including, among others, two strong, symmetrical O · · · H, H · · · O interactions with disordered proton locations. Symmetrically related PO4 anions linked through these protons form infinite (PO4)∞ chains along the crystal a-axis. The anomalies in the temperature dependence of the electric permittivity showed that the crystal undergoes ferroelectric phase transition at Tc = 103.5 K. The spontaneous polarization takes place along the crystal a-axis, being parallel to the chains of the hydrogen-bonded PO4. The disordered protons, thermally activated at room temperature, can be frozen at their positions in the ferroelectric phase. The order–disorder continuous type of the transition has been evidenced on the basis of the temperature dependences of electric permittivity and spontaneous polarization measurements.

Characterization of phase transition in new ferroelectric 2‐aminopyridine phosphate crystal

“…3.1 Temperature dependence of electric permittivity in the paraelectric phase As demonstrated previously [1] at the phase transition, the dielectric constant gains the value of some tens of thousands. In the experimental conditions, the dielectric constant and its reciprocal value as a function of temperature are presented in Fig.…”

“…The 2-aminopyridine phosphate (NC 4 H 4 NH 2 ⋅ H 3 PO 4 ), abbreviated as 2-APP, is a new hydrogen-bonded crystal exhibiting the ferroelectric phase transition at 103.5 K [1]. At room temperature, 2-APP belongs to a monoclinic space group C2/c with eight molecular units per unit cell [1].…”

“…At room temperature, 2-APP belongs to a monoclinic space group C2/c with eight molecular units per unit cell [1]. The phase transition is most probably connected with a change of space group C2/c to Cc.…”

“…At 80 K, the value of spontaneous polarization is equal to 8.7 × 10 -3 C/m 2 . The molecular mechanism of the phase transition seems to be related to ordering of protons in hydrogen bonds linking PO 4 tetrahedra into infinite chains along the a-axis [1]. We measured the electrical permittivity as functions of temperature and dc electric field and the temperature dependence of the spontaneous polarization, in order to interpret the dielectric properties of 2-APP crystal by using the classical dielectric equation of state.…”

Crystal structure of the salt 1,1′-(ethane-1,2-diyl)bis(1,4-diazabicyclo[2.2.2]octan-1-ium) diperchlorate, C14H28N4(ClO4)2

“…Organic-inorganic crystalline materials can be exploited for many purposes such as nonlinear optical materials, dielectric, piezoelectric, pyroelectric, ferroelectric, semiconductors and other physical properties [4][5][6][7]. …”