Elastico-mechanoluminescence and crystal-structure relationships in persistent luminescent materials and II–VI semiconductor phosphors B.P. Chandra a,n,1 , V.K. Chandra b , Piyush Jha c a School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur 492010, Chattisgarh, India b Department of Electrical and Electronics Engineering, Chhatrapati Shivaji Institute of Technology, Shivaji Nagar, Kolihapuri, Durg 491001, Chattisgarh, India c Department of Applied Physics, Raipur Institute of Technology, Chhatauna, Mandir Hasuad, Raipur 492101, Chattisgarh, India article info Article history: Received 2 January 2015 Accepted 26 January 2015 Available online 28 January 2015 Keywords: Mechanoluminescence Triboluminescence Sensors Persistent luminescent materials II–VI semiconductors abstract Elastico-mechanoluminescence (EML) has recently attracted the attention of a large number of re- searchers because of its potential in different types of mechano-optical devices. For understanding the mechanism of EML the relationships between elastico-mechanoluminescence (EML) and crystal-struc- ture of a large number of persistent luminescent materials and II–VI semiconductor phosphors known to date are investigated. It is found that, although most of the non-centrosymmetric crystals exhibit EML, certain non-centrosymmetric crystals do not show EML. Whereas, many centrosymmetric crystals do not exhibit EML, certain centrosymmetric crystals exhibit EML. Piezoelectric ZnS:Cu,Cl single crystals do not show EML, but piezoelectric ZnS:Cu,Cl microcrystalline phosphors show very intense EML. Piezoelectric single crystals of undoped ZnS do not show EML. It seems that EML is related to local piezoelectrification near the impurities in crystals where piezoelectric constant is high. Suitable piezoelectric field near the local piezoelectric region and stable charge carriers in traps are required for appearance of EML. The EML of persistent luminescent materials and II–VI semiconductor phosphors can be understood on the basis of piezoelectrically-induced trap-depth reduction model of EML. Using suitable dopants both in non- centrosymmetric and centrosymmetric crystals intense elastico-mechanoluminescent materials emitting desired colours can be tailored, which may find applications in several mechano-optical devices. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Recently, elastico-mechanoluminescence (EML), the lumines- cence induced by elastic deformation of solids, has attracted the attention of a large number of researchers because of its potential in different types of mechano-optical devices such as stress sen- sors, impact sensor, fracture sensor, torsion sensor, friction sensor, damage sensor, real-time visualization of stress distribution in solids, real-time visualization of stress distribution near crack-tip, real-time visualization of quasidynamic crack-propagation in so- lids, mechanoluminescence (ML)-based health monitoring system of structures, novel ML-driven photocell system, ML light sources, ML displays, visualization of internal defects in a pipe, stress imaging of the orthopaedic prosthetic devices such as the artificial legs, determination of ultrasonic power, radiation dosimetry, sensing the formation and propagation of Lüders bands, visuali- zation of Portevin–Le Chatelier effect, etc. [1–10]. Jeong et al. [8– 10] have demonstrated that the composite film containing ZnS:Cu, Mn and ZnS:Cu phosphors in a polydimethylsiloxane (PDMS) matrix can be used to produce light source, multicolour displays, and wind-driven light sources. The elastico-mechanoluminescent materials present an accu- rate linearity of EML intensity against pressure in the elastic region and they offer the advantages of repetitive, wireless, non-de- structive, reproducible, real-time and reliable stress sensing. The processes responsible for the EML excitation in noncentrosym- metric and centrosymmetric crystals are not satisfactorily known till now, and therefore, it is difficult to tailor the intense elastico- mechanoluminescent materials emitting desired colours. The persistent luminescent materials and II–VI semiconductor phos- phors have been reported to be prominent elastico-mechan- oluminescent materials. The present paper explores the elastico- mechanoluminescence and crystal-structure relationships in per- sistent luminescent materials and II–VI semiconductor phosphors known to date and shows that most of the noncentrosymmetric crystals and many centrosymmetric crystals exhibit EML. It is shown that the local piezoelectric field near the defects in crystals is responsible for the EML emission in the crystals. It is to be noted Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B http://dx.doi.org/10.1016/j.physb.2015.01.030 0921-4526/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: bpchandra4@yahoo.co.in (B.P. Chandra). 1 Emeritus Professor. Physica B 463 (2015) 62–67