[PDF] Behavior of H2O surrounding NH4+ and Al3+ in NH4Al(SO4)2

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Behavior of H

2

O surrounding NH

4+ and Al 3+ in NH 4 Al(SO 4 2 $12H 2 Oby 1

H MAS NMR,

14

N NMR, and

27

Al NMR

Ae Ran Lim

ab In order to understand the thermodynamic properties and structural geometry of NH 4 Al(SO 4 2 $12H 2 O, we

studied the magic angle spinning nuclear magnetic resonance (MAS NMR) and static NMR as a function of

temperature. The changes of the chemical shifts, linewidths, resonance frequencies, and spin-lattice relaxation times for 1 H, 14

N, and27

Al nuclei nearT

d (¼335 K), which is interpreted as the onset of partial thermal decomposition, are due to the structural phase transition. The changes nearT d are related to the loss of H 2 O, which probably disrupts the forms of the octahedra of water molecules surrounding NH 4+ and Al 3+ ions. This mechanism aboveT d is related to hydrogen-bond transfer involving the breakage of the weak part of the hydrogen bond.

I. Introduction

The alums can be represented by the general formula A I B III (SO 4 2 $12H 2

O, where A is a monovalent cation such as Na,

K, Rb, Cs, or NH

4 , and B is a trivalent cation such as Al, Fe, or Cr. 1-11 It is well known that there are a considerable number of alums A I B III (SO 4 2 $12H 2

O that exhibit ferroelectric activity.

12 The twelve water molecules are located in an octahedral coordination in two crystallographically-different areas surrounding the A I cation and the B III ion complex.A complex network of H-bonds is one of the main features of alum structures. Current interest in these compounds lies in the fact that a number of them, partic- ularly when A

Iis NH

4 , are ferroelectric at low temperature. NH 4 Al(SO 4 2 $12H 2

O crystals, one of the A

I B III (SO 4 2 $12H 2

O types,

haveacubicstructureandbelongtothespacegroupPa?3 withZ¼

4 per unit cell at room temperature. The lattice constants of

NH 4 Al(SO 4 2 $12H 2 13

The NH

4+ and Al 3+ ions in the NH 4 Al(SO 4 2 $12H 2

O crystal are each sur-

rounded by six water molecules, as shown in Fig. 1. As in other alums, each trivalent Al 3+ is surrounded by an almost regular octahedron of six water molecules, whereas the remaining six water molecules surrounding the NH 4+ cation form a highly distorted octahedron. In the case of NH 4 Al(SO 4 2 $12H 2

O, the

bond-length of Al-6H 2 6H 2 14

The phase transition temperature of NH

4 Al(SO 4 2 $12H 2 O crystals at low temperature has been reportedto be 58 K and71K on cooling and heating, respectively. 9,13

According to previousreports, NH

4 Al(SO 4 2 $12H 2

O melts congruently at 367.13 K with

a phase transition enthalpy of 122.2 kJ mol ?1 11

In addition, the

phase transitiontemperatures were reported as 370 K,390 K,and

495 K using differential scanning calorimetry (DSC) by our

group. 15 Furthermore, the mass loss begins in the vicinity of 335

K(¼T

d ), which is interpreted as the onset of partial thermal decomposition. On the other hand, the quadrupole coupling constants for 14 Nand 27

Al in NH

4 Al(SO 4 2 $12H 2

O crystals have

been reported to be 202 kHz and 446 kHz at room temperature, respectively. 16-19 The 1

Hspin-lattice relaxation timeT1

in the laboratory frame and spin-lattice relaxation timeT 1r in the rotating frame of NH 4 Al(SO 4 2 $12H 2

O below 200 K have been

studied by Svareet al., 20 and were explained in terms of the hopping of NH 4+ between two positions with different orienta- tions and electron dipole moments. 19

In addition, the nuclear

magnetic resonance (NMR) spectrum and the spin-lattice relax- ation times in the laboratory frame for 1 Hand 27

Al of NH

4 Al(SO 4 2 $12H 2

O crystals above 180 K were previously reported;

the changes in the temperature dependences of the relaxation times nearT d were related to the loss of H 2

O, which probably

disrupts the forms of the octahedral of water molecules surrounding Al 3+ 15

Although these interesting properties have

been studied by many methods, those related to the thermody- namic propertiesathightemperaturehave not yet been reported. In the present study, the chemical shi, linewidth, and spin- lattice relaxation time,T1r , in the rotating frame of NH 4 Al(SO 4 2 $12H 2

O were measured as a function of temperature

using 1 Hmagic angle spinning (MAS) NMR, focusing on the role of NH 4 and H 2

O. In addition, the

14

N NMR spectra in NH

4 Al(SO 4 2 $12H 2

O single crystals as a function of temperature

were discussed in order to elucidate the structural geometry. At high temperature, we use these results to analyze the behavior of NH 4+ and Al 3+ surrounded by six water molecules, a Analytical Laboratory of Advanced Ferroelectric Crystals, Jeonju University, Jeonju

55069, Korea

b Department of Science Education, Jeonju University, Jeonju 55069, Korea. E-mail: aeranlim@hanmail.net; arlim@jj.ac.kr

Cite this:RSC Adv.,2017,7,55276

Received 27th August 2017

Accepted 30th November 2017

DOI: 10.1039/c7ra09488d

rsc.li/rsc-advances

55276|RSC Adv.,2017,7, 55276-55281This journal is © The Royal Society of Chemistry 2017

RSC Advances

PAPER Open Access Article. Published on 05 December 2017. Downloaded on 10/8/2023 7:42:54 AM.

This article is licensed under a

Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal | View Issue respectively, from the results of 1

H MAS NMR,

14

N NMR here

obtained, and the 27

Al NMR previously reported. The connec-

tion between the crystal structure and the thermal properties of NH 4 Al(SO 4 2 $12H 2

O is discussed.

II. Experimental procedure

Single crystals of NH

4 Al(SO 4 2 $12Hquotesdbs_dbs35.pdfusesText_40

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